Add support for registering custom casts (and types) through c api

[Maxxen]

This PR is a follow-up that contains #13490 but also adds new functions for registering custom cast functions.

These work similarly to scalar functions and provide similar capabilities to set e.g. custom user data. But compared to implementing cast functions in C++, the c-api also provides the possibility to detect whether or not the cast is executed within a TRY_CAST. Additionally it won't throw an exception in a non-try cast as soon as you set the error message (as would be the case if you used c++'s HandleCastError util). This allow c-based cast functions to be able to clean up any temporary resources when an invalid cast input is encountered during a non-try cast, while also being able to explicitly short-circuit execution by simply returning false when they are ready after setting the error message.

[Mytherin]

Thanks for the PR! Looks good - one comment:

[Mytherin]

Maybe we can have a helper function for this similar to how we handle this in the C++ functions, e.g.:

void (*duckdb_cast_function_set_row_error)(duckdb_function_info info, const char *error, idx_t index, duckdb_vector output);

[Maxxen]

So this would basically invalidate the validity at the current row and set the error message (if not already set) in the same call? I worry that if the user is doing some sort of string formatting to create the error message they are going to perform that work unnecessarily for each invalid row after the first one in the case of a try cast, but maybe that's something they can guard against themselves (by checking the cast mode and passing nullptr for subsequent errors) if that's the case.

[Mytherin]

Yes, in the C++ layer templating takes care of that - but this would require some user handling if they want to avoid that cost. That being said I still think it's cleaner than having to essentially duplicate this code in every cast function - and also less error prone.

[Maxxen]

Alright, do I keep the original cast_function_set_error()?

[Mytherin]

I think that's fine yeah, there might be situations where we want to return errors from a try_cast as well.

[Maxxen]

@Mytherin addresses your feedback

[Mytherin]

Thanks!