Add `flow_run_url` to flow run notification template variables

[zangell44]

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• [X] I added a descriptive title to this issue.
• [X] I used the GitHub search to find a similar request and didn't find it.
• [X] I searched the Prefect documentation for this feature.

Prefect Version

2.x

Describe the current behavior

Flow runs notifications currently allow the following fields to be provided as template variables

FLOW_RUN_NOTIFICATION_TEMPLATE_KWARGS = [
    "flow_run_notification_policy_id",
    "flow_id",
    "flow_name",
    "flow_run_url",
    "flow_run_id",
    "flow_run_name",
    "flow_run_parameters",
    "flow_run_state_type",
    "flow_run_state_name",
    "flow_run_state_timestamp",
    "flow_run_state_message",
]

Describe the proposed behavior

Add flow_run_url to the allowed template variables.

The FlowRunNotifications service will be responsible for figuring out the correct flow run url and adding it to the templated variables.

Example Use

If my flow run fails, I would like a slack notification with a link to the flow run ui page.

Additional context

No response