Custom runtimes to define function fork

[sobolk]

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• If this PR includes a functional change to the runtime behavior of the code, I have added or updated automated test coverage for this change.
• If this PR requires a change to the Project Architecture README, I have included that update in this PR.
• If this PR requires a docs update, I have linked to that docs PR above.
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🦋 Changeset detected

Latest commit: e2f1501

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@aws-amplify/backend-function	Minor
@aws-amplify/backend	Minor

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