template: allow dynamic config value lookup

[fanzeyi]

Fix: #9144

It looks like there is a comment talking about allowing dynamic configuration lookup for this function already. This PR implements that.

I would like to be able to show GitHub PR numbers that I store in my JJ configs. This allows me to write templates for jj log that shows PR numbers.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added/updated tests to cover my changes
• I fully understand the code that I am submitting (what it does,
  how it works, how it's organized), including any code drafted by an LLM.
• For any prose generated by an LLM, I have proof-read and copy-edited with
  an eye towards deleting anything that is irrelevant, clarifying anything
  that is confusing, and adding details that are relevant. This includes,
  for example, commit descriptions, PR descriptions, and code comments.