Was facing issues while writing unit tests for federation code. Mocks weren't catching all network calls, because was being out of scope of the mocking infra. Plus, I think we can have more granular tests.
This PR puts the client behind an interface, that can be retrieved from `ctx`. Context doesn't require initialization, as it defaults to the implementation available in-tree. It may be overridden when required (like testing).
## Mechanism
1. Get client factory from `ctx` (factory contains network and crypto parameters that are needed)
2. Initialize client with sender's keys and the receiver's public key
3. Use client as before.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/4853
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Aravinth Manivannan <realaravinth@batsense.net>
Co-committed-by: Aravinth Manivannan <realaravinth@batsense.net>
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