Fix how conditions are handled in google_bigtable_instance_iam_* resources

[SarahFrench]

This PR

1. Converts existing BigTable IAM acceptance tests to use the context pattern
2. Updates BigTable IAM code to use policy version 3
3. Adds new acceptance tests showing conditions can be used and imported

Release Note Template for Downstream PRs (will be copied)

bigtable: fixed an error where BigTable IAM resources could be created with conditions but the condition was not stored in state

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 3 files changed, 296 insertions(+), 57 deletions(-))
google-beta provider: Diff ( 3 files changed, 296 insertions(+), 57 deletions(-))

[modular-magician]

Tests analytics

Total tests: 49
Passed tests: 0
Skipped tests: 49
Affected tests: 0

Click here to see the affected service packages

• bigtable

#### Non-exercised tests

Tests were added that are skipped in VCR:

• TestAccBigtableInstanceIamBinding_withCondition
• TestAccBigtableInstanceIamMember_withCondition
• TestAccBigtableInstanceIamPolicy_withCondition
  🟢 All tests passed!

View the build log

[SarahFrench]

Here's a build running the 3 skipped tests mentioned above: https://hashicorp.teamcity.com/buildConfiguration/TerraformProviders_GoogleCloud_GOOGLE_MMUPSTREAMTESTS_GOOGLE_PACKAGE_BIGTABLE/243380?hideTestsFromDependencies=false&hideProblemsFromDependencies=false&expandBuildChangesSection=true

[SarahFrench]

☝️ The 3 tests passed in TeamCity! Opening for review.

[zli82016]

LGTM. Thanks for adding the tests.