Add DataPayload::dynamic_cast with example

[sffc]

I came across this in the context of the transliteration example. Adding this function makes forking providers easier to write.

[robertbastian]

This should be applied to the tutorials:

icu4x/tutorials/data_provider.md

Lines 218 to 219 in 1426a7e

	let mut any_payload = (&mut res.payload) as &mut dyn Any;
	if let Some(mut decimal_payload) = any_payload.downcast_mut::<DataPayload<DecimalSymbolsV1Marker>>() {

[robertbastian]

Everywhere we compare data markers types we compare M::INFO. Comparing TypeId::of::<M>() is a new way to do this and we should agree on one.

[sffc]

Hmm, interesting observation.

If we have access to both marker types, comparing TypeId is probably more efficient. There might still be cases where we have a DynamicDataMarker and we have to compare the INFO.

[sffc]

This should be applied to the tutorials:

downcast_mut and dynamic_cast_mut still exist. This PR adds dymamic_cast, which is behavior that didn't exist in the library before.

[robertbastian]

Right, the tutorial should use dynamic_cast_mut

[robertbastian]

#5490