Proto: contract-compatible encodings for implicit and originated accounts

Context

The encoding of Contract or Destination is sometimes used when we only needed an implicit or originated account.

This MR provides encodings for them that are backward-compatible for the one of Contract. I should be used for the storage to avoid a migration or operations for which we don't want to change the encoding.

They are also the de-facto encoding to use for forward-compatibility, e.g. if you build something that works only for originated accounts but may want to later support implicit accounts too.

Previous related MR: !5188 (merged)

Manually testing the MR

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See !5224 (merged) for a usage of it

Checklist

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