feat: add support for generic subscriptions

[bpapaspyros]

Description

Along with the changes in #141 and #143, this PR aims to introduce generic subscriptions.

Pending:

• add a couple more tests for callbacks in subscriptions

• n/a TODO

This PR solves the issue by propagating the new concept-related compile-time conditions to add_input, and restructuring the MessagePair::read_message to perform compile-time inference about the functions that need to be used for the given MsgT.

Review guidelines

Estimated Time of Review: 15 minutes

Checklist before merging:

• Confirm that the relevant changelog(s) are up-to-date in case of any user-facing changes

Related issues

Blocked by:

• feat!: add support for generic outputs #141
• feat: expose generic outputs to components #143

Blocks:

• add_output for generic message types #133