Add OpAMP-Instance-UID request header (#284)

Resolves #270

The request header can be used by load balancers to implement
sticky connections and direct traffic from a particular Agent
instance to the same server, allowing for potential performance
optimizations.

The header is introduced as a SHOULD and not a MUST for the following
reasons:

- It allows us to avoid breaking compatibility.
  Existing implementations that don't set the header are still
  considered compliant.

- Reinforced the that servers cannot rely on guarantee that the
  header will be set or that the connection stickiness will be
  guaranteed and thus the server need to be prepared that
  connection stickiness is only performance optimization but
  not a 100% guarantee.

We are not using "X-" prefix for custom header. This practice was deprecated by https://datatracker.ietf.org/doc/html/rfc6648

Author: tigrannajaryan

specification.md

@@ -510,6 +510,13 @@ The Client MAY compress the request body using gzip method and MUST specify
 "Content-Encoding: gzip" in that case. Server implementations MUST honour the
 "Content-Encoding" header and MUST support gzipped or uncompressed request bodies.
 
+The Client SHOULD set "OpAMP-Instance-UID" request header to the value of the
+instance_uid field encoded in
+[canonical string representation](https://datatracker.ietf.org/doc/html/rfc4122#section-3)
+of the UUID (e.g. "OpAMP-Instance-UID: f81d4fae-7dec-11d0-a765-00a0c91e6bf6").
+The value is equal to [instance_uid](#agenttoserverinstance_uid) field in AgentToServer
+message.
+
 The Server SHOULD compress the response if the Client indicated it can accept compressed
 response via the "Accept-Encoding" header.