Add client-initiated certificate request flow (CSR)

Resolves #13

Uses [Development] label as the indication of the least
mature level proposed in this upcoming OTEP:
open-telemetry/oteps#232

specification.md

@@ -1483,6 +1483,8 @@ without disrupting the access to all other Agents.
 
 #### Agent-initiated CSR Flow
 
+Status: [Development]
+
 This is an Agent-initiated flow that allows the Client to send a Certificate
 Signing Request (CSR) to the Server and obtain a self-signed or CA-signed client
 certificate that the Client can use for subsequent OpAMP connections.
@@ -1559,6 +1561,8 @@ to `ServerErrorResponseType_BadRequest`.
 
 ##### Using instance_uid in the CSR
 
+Status: [Development]
+
 The implementation may choose to use Agent's instance_uid as one of the CSR fields
 (or part of the field) and the Server may in such implementations verify that the
 connecting Agent's instance_uid in the payloads matches the certificate's content.