feat: add self signed jwt support

[arithmetic1728]

Add UseJwtAccessWithScope property to GoogleCredentialsProvider, which passes the property value to ServiceAccountCredentials to trigger self signed JWT.

This property will be set by GAPIC clients, example PR: arithmetic1728/java-kms#3

[suztomo]

Jul 21, 2021 5:57:02 AM com.google.cloud.tools.dependencies.linkagemonitor.LinkageMonitor run
SEVERE: Newly introduced problem:
(com.google.auth:google-auth-library-oauth2-http:0.26.0) com.google.auth.oauth2.ServiceAccountCredentials's method createWithUseJwtAccessWithScope(boolean) is not found
  referenced from com.google.api.gax.core.GoogleCredentialsProvider (com.google.api:gax:1.67.1-SNAPSHOT)

com.google.auth:google-auth-library-oauth2-http:0.26.0 is at:
  com.google.auth:google-auth-library-oauth2-http:0.26.0 (compile)
  and 96 other dependency paths.
com.google.api:gax:1.67.1-SNAPSHOT is at:
  com.google.api:gax:1.67.1-SNAPSHOT (compile)
  and 2 other dependency paths.

Linkage Monitor checks linkage errors in the following step:

• It creates a temporary BOM based on the latest release of libraries-bom (20.8.0). It modifies the version of gax with this 1.67.1-SNAPSHOT because this repository generates the artifacts.
  This doesn't modify google-auth-library-oauth2-http version, keeping version 0.26.0, because this repository doesn't have it.
• It runs linkage checker for the class path generated from the temporary BOM
  The class path contains GAX 1.67.1-SNAPSHOT and google-auth-library-oauth2-http 0.26.0.

Because GAX 1.67.1-SNAPSHOT uses a new method in google-auth-library-oauth2-http 0.27.0. It's a new linkage error.

I'm thinking how to resolve this. (GoogleCloudPlatform/cloud-opensource-java#2137)

For now, it requires a new release of the shared dependencies BOM and the Libraries BOM before merging this change.

[arithmetic1728]

Added do not merge while we are waiting for auth lib to be updated in library-bom

[TimurSadykov]

LGTM

[suztomo]

I just released a new version of the Libraries BOM that contains the auth library 1.0.0. It may take time (few hours) for Maven to show it as the latest version.