feat: Add support for persistent copilot threads and messages (#11489)

The agents can see the previous conversations with the copilot if needed
with this change. We would have to cleanup the data after a while. For
now, that is not considered.

This PR adds:
- A new model for copilot_threads (intentionally named thread instead of
conversation to avoid confusion), copilot_messages
- Add the controller to fetch previous threads and messages.

enterprise/app/models/copilot_thread.rb

@@ -0,0 +1,26 @@
+# == Schema Information
+#
+# Table name: copilot_threads
+#
+#  id         :bigint           not null, primary key
+#  title      :string           not null
+#  uuid       :uuid             not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#  account_id :bigint           not null
+#  user_id    :bigint           not null
+#
+# Indexes
+#
+#  index_copilot_threads_on_account_id  (account_id)
+#  index_copilot_threads_on_user_id     (user_id)
+#  index_copilot_threads_on_uuid        (uuid) UNIQUE
+#
+class CopilotThread < ApplicationRecord
+  belongs_to :user
+  belongs_to :account
+  has_many :copilot_messages, dependent: :destroy
+
+  validates :title, presence: true
+  validates :uuid, presence: true, uniqueness: true
+end