Summary:
This diff moves definitions for YBCGetGFlags() to its own file (ybc_gflags.cc and
ybc_gflags.h) as per review comments on D45834.
Jira: DB-17903

Test Plan: Jenkins: all tests

Reviewers: skumar, mkumar, arpit.saxena, asrinivasan, jason

Reviewed By: jason

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D45971