Use public httpsig library in AS for tests

[njlie]

Use publically maintained httpsig library to generate signatures & signature inputs for tests.

• Content-Digest generation does not seem to be in the scope of this package, so the AS testing helpers should still handle that.
  • https://github.com/interledger/httpbis-digest-headers

[wilsonianb]

We could export this from open-payments to use in auth:

rafiki/packages/open-payments/src/client/signatures.ts

Lines 26 to 30 in 461fb76

	export const createSignatureHeaders = async ({
	request,
	privateKey,
	keyId
	}: SignOptions): Promise<SignatureHeaders> => {

I'm currently throwing together a branch to test the upcoming version of http-message-signatures, which adds signature verification and ed25519 support:

• Maturing the API dhensby/node-http-message-signatures#27

[wilsonianb]

We could open a new issue to use something like 👇 in http-signature-utils for signature verification

• feat(open-payments): add verifySignatureHeaders #752