docstore/gcpfilestore: add support for non-default databases.

[coryschwartz]

Fixes: #3339

This edits the regex to allow non-default database names when creating Firestore collections.

In line with prior art, this also creates a helper function to assist with creating a valid scheme.

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[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (6d5d289) 77.47% compared to head (ff1f875) 77.44%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
docstore/gcpfirestore/fs.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3345      +/-   ##
==========================================
- Coverage   77.47%   77.44%   -0.03%     
==========================================
  Files         104      104              
  Lines       13933    13935       +2     
==========================================
- Hits        10794    10792       -2     
- Misses       2378     2382       +4     
  Partials      761      761              

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