Define `OpaqueValue` (#4550)

Define `OpaqueValue` and use it instead of
`grandpa::OpaqueKeyOwnershipProof` and `beefy:OpaqueKeyOwnershipProof`

Related to
https://github.com/paritytech/polkadot-sdk/pull/4522#discussion_r1608278279

We'll need to introduce a runtime API method that calls the
`report_fork_voting_unsigned()` extrinsic. This method will need to
receive the ancestry proof as a paramater. I'm still not sure, but there
is a chance that we'll send the ancestry proof as an opaque type.

So let's introduce this `OpaqueValue`. We can already use it to replace
`grandpa::OpaqueKeyOwnershipProof` and `beefy:OpaqueKeyOwnershipProof`
and maybe we'll need it for the ancestry proof as well.

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Co-authored-by: Bastian Köcher <git@kchr.de>