[Core] Redesign `actor.proto` to handle actor specific parameters

[Olshansk]

Objective

A clear & modular to handle and maintain differences and commonalities between different actor protobuf types.

Origin Document

In #550, we tended #313 which consolidated ActorSpecificParam and GenericParam into ServiceURL while getting rid of MaxRelays. The new actor proto type is:

message Actor {
  ActorType actor_type = 1;
  string address = 2;
  string public_key = 3;
  repeated string chains = 4; // Not applicable to `Validator` actors
  string service_url = 5; // Not applicable to `Application` actors
  string staked_amount = 6;
  int64 paused_height = 7;
  int64 unstaking_height = 8;
  string output = 9;
}

However, service_url is not pertinent to Applications; the field exists but is unused which can lead to confusion and lack of clarity throughout the codebase while adding hidden techdebt.

Goals

• Design a method that makes it easy to distinguish between fields common to all actors and those that are actor specific
• Design a method that makes it easy to extend actor functionality (e.g. adding new fields) with minimal efforts (e.g., BaseActor + ValActor` protobuf?)

Deliverable

• [ ] A PR that tends to the Goals above
• [ ] Documentation on:
  • [ ] All the different actor protobuf types and how they're used (if applicable)
  • [ ] How one would go about adding a new field specific to one type of actor

Non-goals / Non-deliverables

• Adding new functionality to any of the existing actors

General issue deliverables

• [ ] Update the appropriate CHANGELOG(s)
• [ ] Update any relevant local/global README(s)
• [ ] Update relevant source code tree explanations
• [ ] Add or update any relevant or supporting mermaid diagrams

Testing Methodology

• [ ] All tests: make test_all
• [ ] LocalNet: verify a LocalNet is still functioning correctly by following the instructions at docs/development/README.md

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Creator: @Olshansk Co-Owners: @deblasis