feat: Unread Count: added api, store refresher, invalidation and events (2/3)[CW-6851] (#14369)

# Pull Request Template

## Description

This is the second PR in a series of PRs for Introducing unread counts
in the sidebar for inboxes and labels.

In this PR:

* added api for unread counts 
* Added the store refresher and invalidation with event listeners
* Added action cable event
* Added specs for the changes

Issue:
https://linear.app/chatwoot/issue/CW-6851/support-unread-conversation-counts

## Type of change

Please delete options that are not relevant.

- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality not to work as expected)
- [ ] This change requires a documentation update

## How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide
instructions so we can reproduce. Please also list any relevant details
for your test configuration.


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Sojan Jose <sojan@pepalo.com>

app/controllers/api/v1/accounts/conversations/unread_counts_controller.rb

@@ -0,0 +1,16 @@
+class Api::V1::Accounts::Conversations::UnreadCountsController < Api::V1::Accounts::BaseController
+  before_action :ensure_unread_counts_enabled
+
+  def index
+    counts = ::Conversations::UnreadCounts::Counter.new(account: Current.account, user: Current.user).perform
+    render json: { payload: counts }
+  end
+
+  private
+
+  def ensure_unread_counts_enabled
+    return if Current.account.feature_enabled?('conversation_unread_counts')
+
+    render json: { error: I18n.t('errors.conversations.unread_counts.feature_not_enabled') }, status: :forbidden
+  end
+end

app/controllers/api/v1/accounts/conversations_controller.rb

@@ -162,6 +162,8 @@ class Api::V1::Accounts::ConversationsController < Api::V1::Accounts::BaseContro
     # rubocop:disable Rails/SkipsModelValidations
     @conversation.update_columns(updates)
     # rubocop:enable Rails/SkipsModelValidations
+
+    ::Conversations::UnreadCounts::Notifier.new(@conversation).perform
   end
 
   def should_update_last_seen?