feat(plugin): rework broadcast plugin

[rustatian]

Reason for This PR

closes: #729
closes: #733
ref: #704

This PR introduces a reworked broadcast plugin, which is now more user friendly. In our previous approach, websockets plugin was an implementation of the broadcast. Now, broadcast features excluded from the websockets plugin and moved into a separate broadcast plugin which any plugin can import (in the plugin's Init func) to receive broadcast messages.

Description of Changes

• Create a new plugin -> broadcast which can be imported to any plugin to receive broadcast messages.

---

Beta architecture:

---

Driver registration schema:

---

Configuration sample:

websockets: # <----- one of possible subscribers
  path: /ws
  broker: default # <------ broadcast broker to use --------------- |
                                                                    |  match
broadcast: # <-------- broadcast entry point plugin                 |
  default: # <----------------------------------------------------- |
     driver: redis
     addrs:  # <---------------------------- Local redis section
         - "localhost:6379"
      db: 3 # <-- for example, special ws DB
  test:
     driver: memory

New proto schema (still in BETA):

syntax = "proto3";

package websockets.v1beta;
option go_package = "./;websocketsv1beta";

message Message {
    string command = 1;
    repeated string topics = 2;
    bytes payload = 3;
}

// RPC request with messages
message Request {
    repeated Message messages = 1;
}

// RPC response (false in case of error)
message Response {
    bool ok = 1;
}

LIMITATIONS

When you develop your own plugin, make sure to NOT duplicate Redis connections within 1 broadcast section:
For example, image you have such broadcast config:

broadcast:
  test:
    driver: redis
    addrs:
      - "localhost:6379" # <----------
  test2:
    driver: redis
    addrs:
      - "localhost:6379" # <----------
  test3:
    driver: memory
  test4:
    driver: memory

If you tried to push a message into the test and test2 broadcast sections, the message will be duplicated, because you used the same Redis connection. This is equal to publish twice into the test or test2 connections.
You either should use 1 global Redis section OR local Redis sections connected to different clusters.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under
the terms of the MIT license.

PR Checklist

[Author TODO: Meet these criteria.]
[Reviewer TODO: Verify that these criteria are met. Request changes if not]

• All commits in this PR are signed (git commit -s).
• The reason for this PR is clearly provided (issue no. or explanation).
• The description of changes is clear and encompassing.
• Any required documentation changes (code and docs) are included in this PR.
• Any user-facing changes are mentioned in CHANGELOG.md.
• All added/changed functionality is tested.

[codecov]

Codecov Report

Merging #732 (2ab22ac) into master (25e0841) will decrease coverage by 0.03%.
The diff coverage is 56.92%.

@@            Coverage Diff             @@
##           master     #732      +/-   ##
==========================================
- Coverage   67.61%   67.57%   -0.04%     
==========================================
  Files          93       94       +1     
  Lines        4817     4824       +7     
==========================================
+ Hits         3257     3260       +3     
- Misses       1146     1151       +5     
+ Partials      414      413       -1     

Impacted Files	Coverage Δ
plugins/kv/drivers/boltdb/driver.go	52.45% <ø> (ø)
plugins/kv/drivers/boltdb/plugin.go	72.22% <ø> (ø)
plugins/kv/drivers/memcached/driver.go	50.00% <ø> (ø)
plugins/kv/drivers/memcached/plugin.go	58.33% <ø> (ø)
plugins/kv/rpc.go	72.72% <ø> (ø)
plugins/memory/kv.go	60.60% <ø> (ø)
plugins/memory/plugin.go	88.23% <ø> (ø)
plugins/redis/kv.go	60.71% <ø> (ø)
plugins/redis/plugin.go	65.00% <ø> (ø)
proto/kv/v1beta/kv.pb.go	42.60% <ø> (ø)
... and 17 more

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