feat: add mtls feature

[arithmetic1728]

Googlers see go/java-gapic-client-mtls for the detailed design doc.

This PR adds mtls logic to gapic generator. It relies on the gax-java mtls change. This PR requires gax-java version >= 1.65.0.

The following new code is added to StubSetting:

/** Returns the default mTLS service endpoint. */
public static String getDefaultMtlsEndpoint() {    return <mtls endpoint>;  }  <=== new code

private static Builder createDefault() {
      Builder builder = new Builder(((ClientContext) null));
      builder.setTransportChannelProvider(defaultTransportChannelProvider());
      builder.setCredentialsProvider(defaultCredentialsProviderBuilder().build()); 
      builder.setInternalHeaderProvider(defaultApiClientHeaderProviderBuilder().build());
      builder.setEndpoint(getDefaultEndpoint());
      builder.setMtlsEndpoint(getDefaultMtlsEndpoint());  // <======== new code
      builder.setSwitchToMtlsEndpointAllowed(true);  // <========= new code
      return initDefaults(builder);
}

[codecov]

Codecov Report

Merging #672 (e23949c) into master (626cb51) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #672      +/-   ##
==========================================
+ Coverage   91.66%   91.68%   +0.01%     
==========================================
  Files         140      140              
  Lines       14697    14727      +30     
  Branches     1051     1051              
==========================================
+ Hits        13472    13502      +30     
  Misses        941      941              
  Partials      284      284              

Impacted Files	Coverage Δ
...apic/composer/comment/SettingsCommentComposer.java	94.11% <100.00%> (+0.17%)	⬆️
...mmon/AbstractServiceStubSettingsClassComposer.java	99.06% <100.00%> (+0.02%)	⬆️

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[miraleung]

LGTM with tests addressed.