bigtable_table and bigtable_gc_policy support long form ids

[megan07]

Added support for full-name/id instance_name value in google_bigtable_table and google_bigtable_gc_policy

Fixes hashicorp/terraform-provider-google#5820

Release Note Template for Downstream PRs (will be copied)

bigtable: Added support for full-name/id `instance_name` value in `google_bigtable_table` and `google_bigtable_gc_policy`

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 4 files changed, 20 insertions(+), 16 deletions(-))
Terraform Beta: Diff ( 4 files changed, 20 insertions(+), 16 deletions(-))