feat: add external protection level, update documentation

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Log from Synthtool

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/Kms/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:264654a37596a44b0668b8ce6ac41082d713f6ee150b3fc6425fa78cc64e4f20
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/kms/artman_cloudkms.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-kms-v1.
synthtool > Replaced "'serviceAddress' =>" in src/V1/Gapic/KeyManagementServiceGapicClient.php.
synthtool > Replaced '@type string \\$serviceAddress\\n\\s+\\*\\s+The address' in src/V1/Gapic/KeyManagementServiceGapicClient.php.
synthtool > Replaced '\\$transportConfig, and any \\$serviceAddress' in src/V1/Gapic/KeyManagementServiceGapicClient.php.
synthtool > Replaced 'final class' in src/V1/ListImportJobsResponse.php.
synthtool > Replaced 'final class' in src/V1/UpdateCryptoKeyVersionRequest.php.
synthtool > Replaced 'final class' in src/V1/ListCryptoKeyVersionsRequest.php.
synthtool > Replaced 'final class' in src/V1/CreateKeyRingRequest.php.
synthtool > Replaced 'final class' in src/V1/PublicKey.php.
synthtool > Replaced 'final class' in src/V1/AsymmetricSignRequest.php.
synthtool > Replaced 'final class' in src/V1/Digest.php.
synthtool > Replaced 'final class' in src/V1/GetCryptoKeyRequest.php.
synthtool > Replaced 'final class' in src/V1/ListCryptoKeysResponse.php.
synthtool > Replaced 'final class' in src/V1/CryptoKeyVersionTemplate.php.
synthtool > Replaced 'final class' in src/V1/GetImportJobRequest.php.
synthtool > Replaced 'final class' in src/V1/UpdateCryptoKeyRequest.php.
synthtool > Replaced 'final class' in src/V1/KeyOperationAttestation.php.
synthtool > Replaced 'final class' in src/V1/ImportCryptoKeyVersionRequest.php.
synthtool > Replaced 'final class' in src/V1/CryptoKey.php.
synthtool > Replaced 'final class' in src/V1/CreateCryptoKeyRequest.php.
synthtool > Replaced 'final class' in src/V1/KeyRing.php.
synthtool > Replaced 'final class' in src/V1/AsymmetricDecryptRequest.php.
synthtool > Replaced 'final class' in src/V1/AsymmetricSignResponse.php.
synthtool > Replaced 'final class' in src/V1/AsymmetricDecryptResponse.php.
synthtool > Replaced 'final class' in src/V1/GetKeyRingRequest.php.
synthtool > Replaced 'final class' in src/V1/DestroyCryptoKeyVersionRequest.php.
synthtool > Replaced 'final class' in src/V1/EncryptResponse.php.
synthtool > Replaced 'final class' in src/V1/CryptoKeyVersion.php.
synthtool > Replaced 'final class' in src/V1/ListKeyRingsResponse.php.
synthtool > Replaced 'final class' in src/V1/GetCryptoKeyVersionRequest.php.
synthtool > Replaced 'final class' in src/V1/ListCryptoKeyVersionsResponse.php.
synthtool > Replaced 'final class' in src/V1/CreateImportJobRequest.php.
synthtool > Replaced 'final class' in src/V1/EncryptRequest.php.
synthtool > Replaced 'final class' in src/V1/RestoreCryptoKeyVersionRequest.php.
synthtool > Replaced 'final class' in src/V1/ListKeyRingsRequest.php.
synthtool > Replaced 'final class' in src/V1/ListImportJobsRequest.php.
synthtool > Replaced 'final class' in src/V1/DecryptResponse.php.
synthtool > Replaced 'final class' in src/V1/ListCryptoKeysRequest.php.
synthtool > Replaced 'final class' in src/V1/GetPublicKeyRequest.php.
synthtool > Replaced 'final class' in src/V1/DecryptRequest.php.
synthtool > Replaced 'final class' in src/V1/CreateCryptoKeyVersionRequest.php.
synthtool > Replaced 'final class' in src/V1/UpdateCryptoKeyPrimaryVersionRequest.php.
synthtool > Replaced 'final class' in src/V1/LocationMetadata.php.
synthtool > Replaced 'final class' in src/V1/ImportJob.php.
synthtool > Replaced 'final class' in src/V1/ImportJob/WrappingPublicKey.php.
synthtool > No replacements made in src/V*/**/*.php for pattern public function ([s|g]\w{3,})Unwrapped, maybe replacement is not longer needed?
synthtool > Replaced 'Copyright \\d{4}' in src/V1/KeyManagementServiceClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/KeyManagementServiceGrpcClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/Gapic/KeyManagementServiceGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V1/KeyManagementServiceClientTest.php.
synthtool > No replacements made in src/V1/Gapic/KeyManagementServiceGapicClient.php for pattern CryptoKey_CryptoKeyPurpose, maybe replacement is not longer needed?
synthtool > Replaced 'will be removed in the next major release' in src/V1/CryptoKeyVersion_CryptoKeyVersionState.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/CryptoKey_CryptoKeyPurpose.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/CryptoKeyVersion_CryptoKeyVersionAlgorithm.php.
synthtool > Replaced 'will be removed in the next major release' in src/V1/CryptoKeyVersion_CryptoKeyVersionView.php.
synthtool > Wrote metadata to synth.metadata.