Serialize decimal quantiles

[carlopi]

Solution is somehow custom-made, open to suggestions on how to make it more similar to the rest of the Serialization code.

Two main changes (independent but both needed):

• Have that DECIMAL and DECIMAL(X,Y) are a match while looking for overloads in src/function/function_set.cpp
• Encode (as a optionally serialisable field) type information (both on function and argument)

This PR will make so that databases that serialize quantile decimal to storage could not be read by previous version of DuckDB. But given the current case is that those statement are illegal to serialize, it should be anyhow be an improvement.

Main improvement I can think of (and that probably I should implement straight away), is moving the integer flag to an enum. There is anything else that needs addressing?

[Mytherin]

Thanks - LGTM: two comments:

[Maxxen]

My only comment is that I think this would be cleaner and more consistent with the rest of the code if it were turned into an inheritance chain. You could then use the serialization generation script to generate this code.

Also turning the "flag" into "type" enum. Remember to run the generate_enum_util script too.

[carlopi]

Thanks @Maxxen and @Mytherin for helping iterating on this.

Agree with @Maxxen that the implementation via inheritance would be cleaner, but I have to say I was not completely comfortable seeing how to do it.