fix(bigtable): sanitize RowRange proto input

[dbolduc]

Sanitize input for the ctor from protos. Hopefully the comment in the code explains why we want to do this.
Also correct the documentation for this class.

I considered deprecating the constructor that takes a proto, but it is too complicated.

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This change is 

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.92%. Comparing base (cd1c047) to head (535cdbf).
Report is 129 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14651   +/-   ##
=======================================
  Coverage   90.91%   90.92%           
=======================================
  Files        1755     1755           
  Lines      133091   133106   +15     
=======================================
+ Hits       121004   121021   +17     
+ Misses      12087    12085    -2     

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[scotthart]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @dbolduc)