Use `Did` type in `messages` crate

[Patrik-Stas]

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• messages https://github.com/hyperledger/aries-vcx/tree/main/aries/messages crate typically types DID attributes (for example in did-exchange ResponseContent message type) as String.
• Use did_parser_nom's https://github.com/hyperledger/aries-vcx/tree/main/did_core/did_parser_nom Did type instead so we parse&verify DIDs upon message deserialization.

[GHkrishna]

Hello @Patrik-Stas I have created a PR #1171 for this.

Apart from the instances resolved in the PR, there are multiple other instances in agent, misc, etc where did is typed as a string.

Do I convert the PR to draft and add those instances as well?