Allow ignoring cache or using a different cache key for a given formatter

[akshaymankar]

Is your feature request related to a problem? Please describe.

When using cabal-fmt, the file can be updated if there are new haskell modules in the project. The cache doesn't seem to understand this.

Describe the solution you'd like

Either allow an option like ignoreCache: <bool> in the formatter config or allow specifying a command to run to get the cache key.

Describe alternatives you've considered

Just running treefmt --no-cache -f <formatter> works, but it is too verbose and needs treefmt to be run twice: once for formatters for which cache works, second time for rest of them.