Add the ability to edit the flow run notification message template from the UI

[zhen0]

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Prefect Version

2.x

Describe the current behavior

Currently notifications created by the UI only offer the ability to send the default message.

Describe the proposed behavior

#6798 adds the ability to add a flow run url to a flow run notification message and also updated the default message. We should make use of that default message to add the ability to template a message from the UI using the default message as a placeholder/example.

Example Use

Users could decide they want to add different information using the available inputs at https://github.com/PrefectHQ/prefect/pull/6798/commits/09ee1b8a6dcd5204f40541db598d4ce269007381 or they could simply change the wording of the message that gets sent as part of their notification.

Additional context

No response