Adding name field in u object for Pinned/System Messages

Description:

I am working on this issue in Embedded chat https://github.com/RocketChat/EmbeddedChat/issues/78. My observation is for the other messages there is a name property in u object. But this name is missing for pinned messages or other system messages. So to fix this the API we are using for pinning messages, we can also add u.name property while saving such messages. This could also be extended for other system messages.

Steps to reproduce:

1. Make changes in pin message API
2. Add a u.name property in the message

Expected behavior:

Embedded chat can easily get the name of use who pinned the message.

Actual behavior:

Currently we are only getting username

Current behaviour

API response of messages for pinned and simple message, in the u object we can see the difference

Trace the pin-message API and the message-saving path described in the issue, then compare the user object on pinned and ordinary messages. Confirm how other system messages are persisted before deciding whether they belong in scope. Done means pinned messages expose the pinner's name in u.name, with the affected message APIs still returning valid responses.