apprunner/service: Make instance_role_arn optional

[gdavison]

Copied from #20149 due to Git shenanigans.

Closes #20145

Output from acceptance testing:

$ make testacc TESTS=TestAccAppRunnerService_ PKG= apprunner

--- PASS: TestAccAppRunnerService_disappears (287.45s)
--- PASS: TestAccAppRunnerService_ImageRepository_runtimeEnvironmentVars (292.23s)
--- PASS: TestAccAppRunnerService_ImageRepository_basic (302.72s)
--- PASS: TestAccAppRunnerService_ImageRepository_instance_NoInstanceRole (304.72s)
--- PASS: TestAccAppRunnerService_tags (366.45s)
--- PASS: TestAccAppRunnerService_ImageRepository_autoScaling (436.97s)
--- PASS: TestAccAppRunnerService_ImageRepository_instance_Update (483.29s)
--- PASS: TestAccAppRunnerService_ImageRepository_encryption (528.42s)
--- PASS: TestAccAppRunnerService_ImageRepository_healthCheck (570.65s)

[github-actions]

This functionality has been released in v3.67.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[github-actions]

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