bigtable: third party dependency exception for rsc.io/binaryregexp

[noahdietz]

Module: bigtable
Usage(s): bigtable/bttest/inmem.go

For the time being, it will be exempt from the third party dep check, but please do one of the following:

A. Remove the dependency by switching to a package own directly by Google, handwriting necessary functionality, or using stdlib
B. Justify the exception and ack the risks, maintaining the exception indefinitely

[bhshkh]

This dependency was added to resolve #1405
I have tried replacing the dependency with other packages but couldn't find a replacement.
Please add this as an exception