[csp-embedded-enforcement] Add support for a 'hash-required' to align with 'nonce-required'

When using CSPEE to enforce nonce based integrity checks the iframe[csp] attribute must contain 'nonce-required' and the child frame response should contain a randomly generated 'nonce-r4nd0m' directive. This change is great since it mitigated the issue of leaking nonces and made it easier to implement.

However, the same is not observed for 'unsafe-hashes' where currently it seems like the iframe[csp] attribute must contain the same hash that the child responder has. There appears to be no support for 'hash-required' that just asks the server to use hashes full stop.

Abdul