feat: add method to set emulator host programmatically (#319)

[Gerschtli]

See for initial discussion: #319

Fixes #319, #210.
Relates to #190.
Similar issues in other SDKs: googleapis/google-cloud-go#1978, firebase/firebase-admin-node#776.

[codecov]

Codecov Report

Merging #336 into master will decrease coverage by 0.10%.
The diff coverage is 37.50%.

@@             Coverage Diff              @@
##             master     #336      +/-   ##
============================================
- Coverage     72.75%   72.64%   -0.11%     
- Complexity     1044     1047       +3     
============================================
  Files            64       64              
  Lines          5512     5535      +23     
  Branches        681      686       +5     
============================================
+ Hits           4010     4021      +11     
- Misses         1289     1301      +12     
  Partials        213      213              

Impacted Files	Coverage Δ	Complexity Δ
...a/com/google/cloud/firestore/FirestoreOptions.java	35.89% <37.50%> (-0.47%)	7.00 <0.00> (ø)
.../com/google/cloud/firestore/CustomClassMapper.java	76.84% <0.00%> (-0.73%)	111.00% <0.00%> (+2.00%)	⬇️
...in/java/com/google/cloud/firestore/BulkWriter.java	71.49% <0.00%> (+0.46%)	29.00% <0.00%> (+1.00%)

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[BenWhitehead]

Apologies on the delayed review, one small request in the javadoc comment but other than it looks good.

[Gerschtli]

Thank you for your input. Updated the JavaDoc.

[BenWhitehead]

Thanks for adding this @Gerschtli!

I was successfully able to use it in a testing project by doing the following:

    final Firestore ds = FirestoreOptions.getDefaultInstance().toBuilder()
        .setEmulatorHost("localhost:8081")
        .setCredentials(new FirestoreOptions.EmulatorCredentials())
        .setProjectId("testing")
        .build()
        .getService();

[Gerschtli]

In fact you should be able to drop the line .setCredentials(new FirestoreOptions.EmulatorCredentials()).

[BenWhitehead]

The credentials line is necessary for some admin related operations, so I always include it.

[Gerschtli]

But the credentials will be overridden in the build method via this.setCredentialsProvider(FixedCredentialsProvider.create(new EmulatorCredentials()));. This should be the same as your credentials or did I miss something?