Add value() and name() methods to enum types. (#1769)

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:
```
synthtool > Executing /tmpfs/src/git/autosynth/working_repo/Bigtable/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:854131ec1af7b3a313253474c24748dc0acd217a58a0b74dbfb559f340a15d78
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/bigtable/artman_bigtable.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-bigtable-v2.
synthtool > No files in sources /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-bigtable-v2/proto/src/Google/Cloud/BigTable were copied. Does the source contain files?
synthtool > Running generator for google/bigtable/admin/artman_bigtableadmin.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-bigtable-admin-v2.
synthtool > Replaced 'Copyright \\d{4}' in src/Admin/V2/Gapic/BigtableTableAdminGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/Admin/V2/Gapic/BigtableInstanceAdminGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/Gapic/BigtableGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V2/BigtableClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/Admin/V2/BigtableTableAdminClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/Admin/V2/BigtableInstanceAdminClientTest.php.
synthtool > Replaced 'Cluster_State' in src/Admin/V2/Gapic/BigtableInstanceAdminGapicClient.php.
synthtool > Replaced 'Instance_Type' in src/Admin/V2/Gapic/BigtableInstanceAdminGapicClient.php.
synthtool > Replaced 'Instance_State' in src/Admin/V2/Gapic/BigtableInstanceAdminGapicClient.php.
synthtool > Replaced 'CreateTableRequest_Split' in src/Admin/V2/Gapic/BigtableTableAdminGapicClient.php.
synthtool > Replaced 'ModifyColumnFamiliesRequest_Modification' in src/Admin/V2/Gapic/BigtableTableAdminGapicClient.php.
synthtool > Replaced 'Table_View' in src/Admin/V2/Gapic/BigtableTableAdminGapicClient.php.
synthtool > Replaced 'MutateRowsRequest_Entry' in src/V2/Gapic/BigtableGapicClient.php.
synthtool > Replaced 'namespace\\ Google\\\\Cloud\\\\Bigtable\\\\Admin\\\\Tests\\\\Unit' in tests/Unit/Admin/V2/BigtableTableAdminClientTest.php.
synthtool > Replaced 'namespace\\ Google\\\\Cloud\\\\Bigtable\\\\Admin\\\\Tests\\\\Unit' in tests/Unit/Admin/V2/BigtableInstanceAdminClientTest.php.
synthtool > Cleaned up 1 temporary directories.
synthtool > Wrote metadata to synth.metadata.

```