skip unnecessary prompt for 'amplify update function'

[johnpc]

Description of changes

When calling amplify update function, the user is prompted to select whether they want to update a lambda function or lambda layer. This prompt should be skipped if only one of the two exist, and it should default as though option that exists was selected.

Issue #, if available

#7308

Description of how you validated changes

• manually tested case where only a function exists
• manually tested case where only a layer exists
• manually tested case where both exist
• manually tested case where neither exists
• added unit tests
• updated e2e test

Checklist

• PR description included
• yarn test passes
• Tests are changed or added

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jhockett]

LGTM!