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Add processing of on extensions in responses in the request manager

define types and mock implementation for supporting extensions

refactor hook interface to pass an actions object

When the request manager receives extensions in a request, it processes them and returns their
response

define types and mock implementation for supporting extensions

add a default validation policy that only accepts selectors up to 100 depth

add the ability to send extensions when creating requests

To reduce future import cycles, and expose more types properly, move the root package to an
interface/types file and move the main implementation into /impl. This also exposes an interface
other implementers can replace.