feat: add self signed jwt feature

[arithmetic1728]

follow up PR of googleapis/google-auth-library-java#572

This PR does the following:
(1) added defaultScopes getter/setter to GoogleCredentialsProvider, and let GoogleCredentialsProvider passes the defaultScopes to ServiceAccountCredentials.
(2) updated google-auth-library-java version

[codecov]

Codecov Report

Merging #1302 (5e3d4ce) into master (49efdc3) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1302      +/-   ##
============================================
+ Coverage     80.34%   80.38%   +0.04%     
- Complexity     1331     1332       +1     
============================================
  Files           210      210              
  Lines          5682     5685       +3     
  Branches        519      519              
============================================
+ Hits           4565     4570       +5     
+ Misses          912      911       -1     
+ Partials        205      204       -1     

Impacted Files	Coverage Δ	Complexity Δ
...google/api/gax/core/GoogleCredentialsProvider.java	87.50% <100.00%> (+1.29%)	6.00 <0.00> (ø)
.../java/com/google/api/gax/batching/BatcherImpl.java	96.06% <0.00%> (+1.12%)	23.00% <0.00%> (+1.00%)

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[arithmetic1728]

@chingor13 I updated google-auth-library-java to 0.25.0 in dependencies.properties in this PR, but the ci / linkage-monitor test is using 0.24.1. How do I fix this?

Mar 17, 2021 12:29:47 AM com.google.cloud.tools.dependencies.linkagemonitor.LinkageMonitor run
INFO: BOM Coordinates: com.google.cloud:libraries-bom:19.0.0
Mar 17, 2021 12:34:26 AM com.google.cloud.tools.dependencies.linkagemonitor.LinkageMonitor run
SEVERE: Newly introduced problem:
(com.google.auth:google-auth-library-oauth2-http:0.24.1) com.google.auth.oauth2.GoogleCredentials's method createScoped(java.util.Collection, java.util.Collection) is not found
  referenced from com.google.api.gax.core.GoogleCredentialsProvider (com.google.api:gax:1.62.1-SNAPSHOT)

[suztomo]

I need to release a new version of the libraries-bom to make the test pass, as Linkage Monitor uses the baseline from the latest release, not from the master branch.

[suztomo]

I released com.google.cloud:libraries-bom:19.2.0. I just merged the master branch to trigger the Linkage Monitor check.

[suztomo]

The version of the auth library breaks the java sample project. Can you check that first?

The fix of the libraries bom 19.2.1 was released