Add retries to datastore index when under contention.

[rileykarson]

Fixes hashicorp/terraform-provider-google#6546

Release Note Template for Downstream PRs (will be copied)

datastore: Added retries to `google_datastore_index` requests when under contention.

[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, 14 insertions(+), 5 deletions(-))
Terraform Beta: Diff ( 4 files changed, 14 insertions(+), 5 deletions(-))
TF Conversion: Diff ( 1 file changed, 9 insertions(+))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=119040"