feat: add ability to partially update retry settings

[igorbernstein2]

The current approach to updating retry settings is very error prone. We've had a few customers accidentally disable retries trying to update the total timeout doing something like:

SomeClientSettings.Builder clientSettingsBuilder = SomeClientSettings.newBuilder();
SomeClientStubSettings.Builder stubSettingsBuilder = clientSettingsBuilder.stubSettings();

stubSettingsBuilder.someMethod()
  .setRetrySettings(
     RetrySettings.newBuilder().setTotalTimeout(Duration.ofSecond(10)).build()
  );

The correct way is not really intuitive and quite verbose:

SomeClientSettings.Builder clientSettingsBuilder = SomeClientSettings.newBuilder();
SomeClientStubSettings.Builder stubSettingsBuilder = clientSettingsBuilder.stubSettings();

stubSettingsBuilder.someMethod()
  .setRetrySettings(
     stubSettingsBuilder.someMethod().getRetrySettings().toBuilder()
        .setTotalTimeout(Duration.ofSecond(10))
        .build()
  );

This PR exposes the underlying RetrySettings.Builder in Unary & ServerStreaming CallSettings. This allows the following pattern:

SomeClientSettings.Builder clientSettingsBuilder = SomeClientSettings.newBuilder();
SomeClientStubSettings.Builder stubSettingsBuilder = clientSettingsBuilder.stubSettings();

stubSettingsBuilder.someMethod()
  .addRetryableCode(Code.DEADLINE_EXCEEDED)
  .retrySettings()
     .setTotalTimeout(Duration.ofSecond(10));

[codecov]

Codecov Report

Merging #993 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #993      +/-   ##
============================================
+ Coverage     78.66%   78.67%   +<.01%     
  Complexity     1167     1167              
============================================
  Files           204      204              
  Lines          5152     5154       +2     
  Branches        413      413              
============================================
+ Hits           4053     4055       +2     
  Misses          925      925              
  Partials        174      174

Impacted Files	Coverage Δ	Complexity Δ
...java/com/google/api/gax/rpc/UnaryCallSettings.java	100% <100%> (ø)	5 <0> (ø)	⬇️
...oogle/api/gax/rpc/ServerStreamingCallSettings.java	67.85% <100%> (+0.58%)	6 <0> (ø)	⬇️

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[vam-google]

In general I believe making nested builders is not salable and error-prone (each level of nested builders needs to propagate its "building" logic all way up to the bottom).

If this comes from the real users experience and really makes their life easier, then probably it is worth doing it. Please check the comments first.

[vam-google]

LGTM

[igorbernstein2]

Thanks for the review! Will merge after presubmits finish

[igorbernstein2]

@vam-google something weird is happening with kokoro...the integration, dependencies, format and binary compat presubmits never finish

[vam-google]

@chingor13 Do you have any ideas what is happening with kokoro?

[chingor13]

The setting configuration for the org got messed up and created extra required checks on repos it should have ignored. We should be able to remove those required checks, but I'm not an admin on this repo so I can't do it.