Add a url_for utility for generating URLs for Prefect objects

[abrookins]

Adds a url_for utility function that generates a URL for any Prefect API object. This gives us a consistent place to generate URLs (something we do in various places) and also gives users a way to generate a UI URL or API URL for API objects, such as FlowRuns, TaskRuns and PrefectFuture objects representing task runs.

Example

Checklist

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[abrookins]

I'm ok with where this landed, but I'll probably want to go back someday and add all the different variations of each schema, since there are a lot of different versions of each one (server vs. client runs, server vs. core event and automations schemas, etc.).

[chrisguidry]

Beautifully done