feat(plugin): implement FetchStageCommands and SendStageNotification for PluginService

[SoumyaRaikwar]

What this PR does:
This PR implements two missing RPC endpoints for the PluginService: FetchStageCommands and SendStageNotification.

• FetchStageCommands allows plugins to retrieve all stage commands for a given deployment by fetching them systematically from the piped command store via a newly added ListStageCommandsByDeployment method.
• SendStageNotification enables plugins to send notifications via the piped component's notifier. It reconstructs notification events by fetching the deployment metadata and shared notification configurations (e.g., mentioned Slack users/groups), restoring the notification capabilities plugins had when they were embedded executors.

Why we need it:
These RPCs were part of the ongoing effort to refactor plugins into a standalone architecture but were left unimplemented. Implementing them is crucial for the PluginService to fully support functionalities like the wait-approval stage, which relies on the ability to fetch all commands to process approvals and send stage notifications to approvers.

Which issue(s) this PR fixes:

Part of #4980

Does this PR introduce a user-facing change?:
No

• How are users affected by this change: N/A
• Is this breaking change: No
• How to migrate (if breaking change): N/A

[SoumyaRaikwar]

@Warashi @khanhtc1202 Please take a look whenever you have chance, Thanks!