Support for RFC 9421, HTTP Message Signatures

### Is your feature request related to a problem? Please describe the problem.

[RFC 9421 HTTP Message Signatures](https://www.rfc-editor.org/rfc/rfc9421) is a mechanism for creating, encoding, and verifying digital signatures or message authentication codes over components of an HTTP message.

It is in use by [ActivityPub clients and servers](https://swicg.github.io/activitypub-http-signature/), like [Mastodon](https://github.com/mastodon/mastodon/issues/21429), and APIs using [RFC 9635 Grant Negotiation and Authorization Protocol (GNAP)](https://www.rfc-editor.org/rfc/rfc9635), like the [Open Payments API specification](https://openpayments.dev/introduction/overview/#grant-negotiation-and-authorization).

### Client library/SDK language

None

### Describe the solution you'd like

I would like a way for the generated SDKs to sign requests and verify responses, as specified by RFC 9421.

### Additional context

Similar request for [curl](https://github.com/curl/curl/discussions/13376)

Start by reading RFC 9421 and reviewing how generated SDKs currently create requests and verify responses. No files or entry points are identified in the issue; done would mean generated SDKs can sign requests and verify responses according to the RFC.

Written by the indexing model from the issue text.