Add Environment Token support

[alextwoods]

[DO NOT MERGE] Currently diffed against WIP authscheme preference branch. This will need to be rebased against master once PR #6083 has been merged.

Motivation and Context

Adds support for configuring bearer auth using a token sourced from the environment.

Modifications

• Adds customization config to enable environment bearer token support.
• When customization flag is set, the BaseClientBuilder will check if the ENV variable is set, and if so, will configure both a prefered auth scheme (using the mechanism from Adding functionality to config preferred authschemeProvider #6083) and add a static token identity provider.
• Includes additional token identity improvements (improve client configuration).

Testing

• New and existing unit tests
• Manual testing against service endpoints.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist

• I have read the CONTRIBUTING document
• Local run of mvn install succeeds
• My code follows the code style of this project
• My change requires a change to the Javadoc documentation
• I have updated the Javadoc documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed
• I have added a changelog entry. Adding a new entry must be accomplished by running the scripts/new-change script and following the instructions. Commit the new file created by the script in .changes/next-release with your changes.
• My change is to implement 1.11 parity feature and I have updated LaunchChangelog

License

• I confirm that this pull request can be released under the Apache 2 license