Add signature headers to AS endpoints

[wilsonianb]

Move signature header parameters to shared schemas.yaml

open-payments/openapi/resource-server.yaml

Lines 1324 to 1339 in 62c4b4a

	signature:
	name: Signature
	in: header
	schema:
	type: string
	example: 'Signature: sig1=:EWJgAONk3D6542Scj8g51rYeMHw96cH2XiCMxcyL511wyemGcw==:'
	description: 'The signature generated based on the Signature-Input, using the signing algorithm specified in the "alg" field of the JWK.'
	required: true
	signature-input:
	name: Signature-Input
	in: header
	schema:
	type: string
	example: 'Signature-Input: sig1=("@method" "@target-uri" "content-digest" "content-length" "content-type");created=1618884473;keyid="gnap-rsa"'
	description: 'The Signature-Input field is a Dictionary structured field containing the metadata for one or more message signatures generated from components within the HTTP message. Each member describes a single message signature. The member''s key is the label that uniquely identifies the message signature within the context of the HTTP message. The member''s value is the serialization of the covered components Inner List plus all signature metadata parameters identified by the label. The following components MUST be included: - "@method" - "@target-uri" - "authorization" When the message contains a request body, the covered components MUST also include the following: - "content-digest" The keyid parameter of the signature MUST be set to the kid value of the JWK. See [ietf-httpbis-message-signatures](https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-message-signatures#section-4.1) for more details.'
	required: true

Include header parameter in relevant grant request(s)

open-payments/openapi/resource-server.yaml

Lines 221 to 223 in 62c4b4a

	parameters:
	- $ref: '#/components/parameters/signature-input'
	- $ref: '#/components/parameters/signature'

open-payments/openapi/auth-server.yaml

Lines 20 to 22 in 62c4b4a

	/:
	post:
	summary: Grant Request

[njlie]

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