chore: make FirestoreException factory methods public to allow construction for testing

[BenWhitehead]

• Expose each factory method at @BetaApi level ensuring no guarantee about api surface
• Expose getStatus() at @BetaApi up from package private @internalapi

[codecov]

Codecov Report

Merging #477 (87bf1a9) into master (d17e17c) will decrease coverage by 0.02%.
The diff coverage is 48.14%.

@@             Coverage Diff              @@
##             master     #477      +/-   ##
============================================
- Coverage     74.45%   74.43%   -0.03%     
+ Complexity     1117     1108       -9     
============================================
  Files            66       66              
  Lines          5853     5855       +2     
  Branches        724      726       +2     
============================================
  Hits           4358     4358              
- Misses         1267     1269       +2     
  Partials        228      228              

Impacted Files	Coverage Δ	Complexity Δ
...va/com/google/cloud/firestore/CollectionGroup.java	18.91% <0.00%> (ø)	1.00 <0.00> (ø)
...om/google/cloud/firestore/CollectionReference.java	43.75% <0.00%> (ø)	10.00 <0.00> (ø)
.../com/google/cloud/firestore/DocumentReference.java	70.32% <0.00%> (ø)	31.00 <0.00> (ø)
...a/com/google/cloud/firestore/FirestoreOptions.java	35.89% <0.00%> (ø)	7.00 <0.00> (ø)
...rc/main/java/com/google/cloud/firestore/Query.java	88.16% <0.00%> (ø)	147.00 <0.00> (-7.00)
...java/com/google/cloud/firestore/UpdateBuilder.java	94.97% <0.00%> (ø)	59.00 <0.00> (ø)
.../com/google/cloud/firestore/UserDataConverter.java	95.37% <0.00%> (-0.90%)	37.00 <0.00> (ø)
...rc/main/java/com/google/cloud/firestore/Watch.java	91.30% <50.00%> (-0.40%)	56.00 <0.00> (ø)
...va/com/google/cloud/firestore/BulkCommitBatch.java	93.05% <100.00%> (ø)	18.00 <0.00> (ø)
.../com/google/cloud/firestore/BulkWriterOptions.java	100.00% <100.00%> (ø)	2.00 <0.00> (ø)
... and 3 more

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