BearerDID getSigner function

[amika-sq]

Depends on part 1

Adds a getSigner function to BearerDID. It can be called in the following way:

let didJWK = try DIDJWK.create(keyManager: InMemoryKeyManager())
let signer = try didJWK.getSigner()

let signature = try signer.sign(payload: payload)
let isValid = try signer.verify(payload: payload, signature: signature)

You can also provide an optional verificationMethodID, which can be used to specify which verification method to use with the signer. If it's an invalid verification method identifier, the function will throw an error. Example usage:

let didJWK = try DIDJWK.create(keyManager: InMemoryKeyManager())
let signer = try didJWK.getSigner(verificationMethodID: "1234")

let signature = try signer.sign(payload: payload)
let isValid = try signer.verify(payload: payload, signature: signature)

[amika-sq]

I'm just defaulting to the first verification method for now, but we could change this.

If we want this to differ based on the DID Method that created this BearerDID, we'd need to do some additional mapping of the did.methodName (which is just a raw string right now) function to it's appropriate DID method.

[amika-sq]

current getSigner pseudo-spec here: decentralized-identity/web5-spec#112