feat: add ability to specify custom headers for individual RPC types

[schmidt-sebastian]

This makes it possible to specify custom headers for a single RPC type as such:

 firestore = new Firestore({commit: {customHeaders: {'X-Header: 'Value'}}});

[codecov]

Codecov Report

Merging #1355 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1355   +/-   ##
=======================================
  Coverage   98.55%   98.55%           
=======================================
  Files          32       32           
  Lines       19290    19291    +1     
  Branches     1387     1388    +1     
=======================================
+ Hits        19011    19012    +1     
  Misses        276      276           
  Partials        3        3           

Impacted Files	Coverage Δ
dev/src/index.ts	97.52% <100.00%> (+<0.01%)	⬆️

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[bcoe]

A test would be nice, but can understand wanting to get this out to test.