expose a way to build a url for a given resource

[voidlily]

Checklist

• I've searched the issue queue to verify this is not a duplicate feature request.
• I've pasted the output of kargo version, if applicable.
• I've pasted logs, if applicable.

Proposed Feature

Expose a way to build a URL for a given resource, such as exposing ctx.UIBaseURL, or a url_for expr function

Motivation

I want to build slack messages for deployment notification and include links without having to hardcode the url host in the message.

Example uses:

"Freight ${{ ctx.targetFreight.name }} is being promoted to Stage <${{ ctx.UIBaseURL }}/project/${{ ctx.project }}/stage/${{ ctx.stage }}|${{ ctx.stage }}>"

"Freight ${{ ctx.targetFreight.name }} is being promoted to Stage <${{ url_for(ctx.stage) }}|${{ ctx.stage }}>" (possibly stage_url_for if they need to be differentiated by type to work properly in expr?)

[voidlily]

kargo version is 1.6.1

[krancour]

Some thought needs to go into how exactly we want to expose this, but in general, I think this is a great idea and relatively easy. If @hiddeco and I can agree on an approach, this would be a good first issue for anyone looking to contribute.