Update documentation and add support for OAuth and OIDC tokens in Tas…

…k HTTP Request. (#1811)

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:
```
synthtool > Executing /tmpfs/src/git/autosynth/working_repo/Tasks/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:d9597f983d1d4e61272c63cb97b7d8f8234da9999526c35d357de3d781f0ec1b
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/tasks/artman_cloudtasks_v2.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-tasks-v2.
synthtool > Running generator for google/cloud/tasks/artman_cloudtasks_v2beta2.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-tasks-v2beta2.
synthtool > Running generator for google/cloud/tasks/artman_cloudtasks_v2beta3.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-tasks-v2beta3.
synthtool > Replaced 'Copyright \\d{4}' in src/V2beta3/CloudTasksClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2beta3/CloudTasksGrpcClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2beta3/Gapic/CloudTasksGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2beta2/CloudTasksClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2beta2/CloudTasksGrpcClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2beta2/Gapic/CloudTasksGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V2beta3/CloudTasksClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V2beta2/CloudTasksClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/System/V2beta3/TasksServiceSmokeTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/System/V2beta2/TasksServiceSmokeTest.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/CloudTasksClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/CloudTasksGrpcClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/Gapic/CloudTasksGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V2/CloudTasksClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/System/V2/TasksServiceSmokeTest.php.
synthtool > Replaced 'Task_View' in src/V2beta3/Gapic/CloudTasksGapicClient.php.
synthtool > Replaced 'Task_View' in src/V2beta2/Gapic/CloudTasksGapicClient.php.
synthtool > Replaced 'Task_View' in src/V2/Gapic/CloudTasksGapicClient.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2beta3/Task_View.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2beta3/Queue_State.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2beta2/Task_View.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2beta2/Queue_State.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Task_View.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Queue_State.php.
synthtool > Replaced '\\\\GPBMetadata\\\\Google\\\\Api\\\\Resource' in metadata/V2/Queue.php.
synthtool > Replaced '\\\\GPBMetadata\\\\Google\\\\Api\\\\Resource' in metadata/V2/Cloudtasks.php.
synthtool > Replaced '\\\\GPBMetadata\\\\Google\\\\Api\\\\Resource' in metadata/V2/Task.php.
synthtool > Cleaned up 0 temporary directories.
synthtool > Wrote metadata to synth.metadata.

```