feat(cloudformation-template): allow to set a static prefix to stack exports

[msessa]

What does this PR do?

Simple non-breaking change to the CloudFormation template to allow users to set a static prefix that will be used in Stack Exports rather than interpolating the stack name.

Motivation

When deploying the template using StackSets, the resulting stacks have random-ish unpredictable names.
This prevents users from relying on predictable Stack Exports to discover the Forwarder Function ARN or the Datadog API Key Secret.

Testing Guidelines

Tested the template with no parameters to ensure no changes.
Tested the template by deploying via StackSet with prefix setting.

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

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
• 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)