Provide Class for HTTP Signature utilities that require an Actor Key

[jojobii-arks]

Issue

• Currently, utilities within hs that utilize an Actor Key rely on mutable variables that are declared on import.
• I'm unsure what the paradigm is called, but having an "HttpSignatureClient" that can be instantiated via "createHttpSignatureClient" would be pretty handy, and could enable use cases such as being able to sign with different keys from the same runtime.

Notes

• This would require an export for each signing method (node, browser).
  • It's a bit of a stretch, but maybe all of those constructors can be wrapped into a single function that can utilize the necessary Crypto method for signing that is implied from the runtime OR defined manually via configuration argument

[randName]

ah, it's a code style that helps with treeshaking (especially for singletons). But I think it doesn't help much with the current code and wrapping them up in a class makes sense.

(evidently my use case was too focused on single user instances and completely forgot that normal servers need to sign for different users 😅)

[randName]

The signer info is now encapsulated in the Signer class. in the future if there are other runtimes with different signing utilities the CoreSigner class can be extended.

import { sendRequest, Signer } from '...'
const res = await sendRequest(req, new Signer(keyId, key))