Bigtable Instance changes

[danawillow]

• instance_type and display_name are now updatable
• don't detect drift on instance_type to prepare for API changes
• get rid of customdiff for development instances because they sort of compare against config but sort of compare against state and I just think we're going to see more problems with it down the line especially if there are upcoming API changes

Release Note Template for Downstream PRs (will be copied)

* bigtable: added update support for `display_name` and `instance_type`

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 2 files changed, 30 insertions(+), 77 deletions(-))
Terraform Beta: Diff ( 2 files changed, 30 insertions(+), 77 deletions(-))