feat: Allow referencing TypedValueParser inValueParserFactory associated type

[9999years]

When searching for relevant discussions and issues I happened upon #4362, which suggests using TypedValueParser::try_map. Unfortunately, this is not suitable for ValueParserFactory implementations that automatically register a TypedValueParser for a given type, as the TryMapValueParser type returned from try_map is not public.

Partial fix for #5065

[epage]

Let's hold off on solutions until we've figured out the problem and how we should be solving it.

[epage]

I've updated the PR to reflect the clearer explanation from #5065.

Technically, this isn't needed. You can create your own type and then call InnerValueParser::new().try_map(...).parse_ref inside of your value parser but exposing this makes things a bit easier.

And technically, this fix is limited because it won't help when the function passed to try_map is unnameable. Associated type impl traits is really what we need. However, there is unlikely to be a case that needs unnameable types.