bigtable: add full GCPolicy object with PolicyType to bigtable's FamilyInfo

[hoangpham95]

Is your feature request related to a problem? Please describe.
We want to read the GC Policy from bigtable from the external TF provider, so that we can set the resource correctly on read. Current implementation only returns a string policy.

Describe the solution you'd like
Instead of return a string policy, return the whole object with policy type.

Describe alternatives you've considered
Return just a proto should works, but it's discouraged to pass proto around.

Additional context
This item is needed for setting Terraform resource on read.