Add hook into SetIdentifier for custom code before ReadOne is called from AdoptedResource.

[mbaijal]

Issue #, if available:
This is currently required as a temporary fix to be able to use AdoptedResource feature for ScalableTarget in the ApplicationAutoscaling Service but is a useful hook to have.

Description of changes:
Adds a hook into SetIdentifier for custom code before ReadOne is called from AdoptedResource.

Sample Usage:
as seen in this PR

set_identifiers_pre_read_one:
       code: r.customSetIdentifierCode(identifier)

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

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[ack-bot]

Hi @mbaijal. Thanks for your PR.

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[vijtrip2]

/ok-to-test

[vijtrip2]

The position of hook is not pre read one. There is no read one call here.

I agree this can be a useful hook. maybe rename it to just "set_identifiers"

Just a thought, Should there be either hook code OR auto-generated code inside method body?
This is such a simple method, no need to have both.

[RedbackThomson]

I believe this is not necessary anymore. The recent replacement with #190 has fixed your use case for applicationautoscaling, and I imagine should fix most other issues. Happy to re-open this PR if hooks are needed in the future.