cloudformation: add parameter for forwarder bucket name

[jimjh]

What does this PR do?

Allow the user to specify a name for the forwarder bucket. This is necessary to avoid a circular dependency when creating permissions boundaries and passed them in via PermissionsBoundaryArn.

From https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-s3-bucket.html

If you don't specify a name, AWS CloudFormation generates a unique ID and uses that ID for the bucket name.

Motivation

Use case: I would like to create a permission boundary that limits datadog S3 privileges, but continue to allow unfettered access to the forwarder bucket (see https://github.com/DataDog/datadog-serverless-functions/blob/master/aws/logs_monitoring/template.yaml#L635-L646).

Before this change, the forwarder bucket's name is always generated, which makes it impossible to create the permission boundary before creating the forwarder bucket/role using cloudformation.

Testing Guidelines

I am forking it and running this within my own CloudFormation pipeline.

Types of changes

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests (should be covered by installation tests.)
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)
• This PR passes the unit tests
• This PR passes the installation tests (ask a Datadog member to run the tests)

[jimjh]

👋 hi can I get some help running these tests?

[tianchu]

@jimjh I was able to test your changes and they worked just fine. Thanks for your contribution!