Add signature headers to AS endpoints

[wilsonianb]

Move signature header parameters to shared schemas.yaml
https://github.com/interledger/open-payments/blob/62c4b4a9875e3adaa21f89f597e88db43016fe0b/openapi/resource-server.yaml#L1324-L1339
Include header parameter in relevant grant request(s)
https://github.com/interledger/open-payments/blob/62c4b4a9875e3adaa21f89f597e88db43016fe0b/openapi/resource-server.yaml#L221-L223
https://github.com/interledger/open-payments/blob/62c4b4a9875e3adaa21f89f597e88db43016fe0b/openapi/auth-server.yaml#L20-L22

[njlie]

Would it also make sense to include adding 401 responses to the AS endpoints for invalid signatures to the scope of this issue?