add the ability to specify app engine database type

[prodaccess]

fixes hashicorp/terraform-provider-google#3657

app_engine: added the option to specify `database_type` in `google_app_engine_application`

[modular-magician]

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[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, 16 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 2 files changed, 16 insertions(+), 1 deletion(-))

[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=120657"

[c2thorn]

Hi @prodaccess, feel free to use hashicorp/terraform-provider-google#6598 as a reference since it was close to complete.

[prodaccess]

Found another PR that was doing the same thing: hashicorp/terraform-provider-google#6598. Referenced for more context.

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 3 files changed, 16 insertions(+), 1 deletion(-))
Terraform Beta: Diff ( 3 files changed, 16 insertions(+), 1 deletion(-))

[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=121142"

[c2thorn]

@prodaccess Looks good to me! Ran some tests and even tried out CLOUD_FIRESTORE successfully. Need to look at one more set of tests, and then we should be good to merge.