Add support for hook endpoint to Index model

[kMutagene]

This PR will add support for optional hook endpoints in validation package metadata. This is compliant with ARC-specification v2.0.0 which defines an optional hook endpoint that can be triggered by validation packages being run.

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[kMutagene]

This needs to be merged with index changes first so we can use the updated package in arcexpect/packages in the staging area