Require that instance_uid is ULID instead of only recommending it (#116)

Resolves #113

This PR changes ULID to be a `MUST` in order to:
- Avoid mistakes when the Agent chooses too short instance id and causes collisions.
- Make the Server's life a bit easier since will know it is always a 128 bits of data and can store in places where UUIDs can be stored (e.g. Postgres has a data type UUID).

specification.md

@@ -390,8 +390,9 @@ The instance_uid field is a globally unique identifier of the running instance
 of the Agent. The Agent SHOULD self-generate this identifier and make the best
 effort to avoid creating an identifier that may conflict with identifiers
 created by other Agents. The instance_uid SHOULD remain unchanged for the
-lifetime of the Agent process. The recommended format for the instance_uid is
-[ULID](https://github.com/ulid/spec).
+lifetime of the Agent process. The instance_uid MUST be a
+[ULID](https://github.com/ulid/spec) formatted as a 26 character string in canonical
+representation.
 
 In case the Agent wants to use an identifier generated by the Server, the field
 SHOULD be set with a temporary value and RequestInstanceUid flag MUST be set.
@@ -660,7 +661,9 @@ enum ServerCapabilities {
 
 Properties related to identification of the Agent, which can be overriden by the
 Server if needed. When new_instance_uid is set, Agent MUST update instance_uid
-to the value provided and use it for all further communication.
+to the value provided and use it for all further communication. The new_instance_uid MUST
+be a [ULID](https://github.com/ulid/spec) formatted as a 26 character string in canonical
+representation.
 
 ```protobuf
 message AgentIdentification {