[AWSX] feat(logs forwarder): update template and document custom source override

[ge0Aja]

What does this PR do?

• Update terraform template to allow setting a custom source which will be used by the forwarded.
• The feature is already there, we're just surfacing this functionality in the template and docs.

Motivation

Align template and docs with forwarder functionality.

Testing Guidelines

Additional Notes

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
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• This PR passes the unit tests
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