feat: add BigtableDataClientFactory to create lightweight data clients

[igorbernstein2]

The new factory allows users to construct a single heavy factory object that can create many lightweight clients.
This is meant to be used in situations when a single application needs to access multiple instances or use different application profiles

[igorbernstein2]

@tonytanger Can you take a look as well? I want to make sure this doesn't conflict with your channel refresh work

[codecov]

Codecov Report

Merging #112 into master will increase coverage by 0.41%.
The diff coverage is 94.11%.

@@             Coverage Diff              @@
##             master     #112      +/-   ##
============================================
+ Coverage     81.33%   81.74%   +0.41%     
- Complexity      937      952      +15     
============================================
  Files            95       97       +2     
  Lines          5850     5927      +77     
  Branches        325      327       +2     
============================================
+ Hits           4758     4845      +87     
+ Misses          916      904      -12     
- Partials        176      178       +2

Impacted Files	Coverage Δ	Complexity Δ
...ble/data/v2/stub/EnhancedBigtableStubSettings.java	94.3% <100%> (+0.55%)	18 <0> (+1)	⬆️
...e/cloud/bigtable/data/v2/BigtableDataSettings.java	67.74% <66.66%> (+35.59%)	7 <0> (+3)	⬆️
...ud/bigtable/data/v2/BigtableDataClientFactory.java	95.55% <95.55%> (ø)	9 <9> (?)
...oud/bigtable/data/v2/models/DefaultRowAdapter.java	96.87% <0%> (-0.05%)	8% <0%> (ø)
...loud/bigtable/data/v2/internal/RefreshChannel.java	40% <0%> (ø)	2% <0%> (?)

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[tonytanger]

LGTM. Don't think primechannel and this will conflict.