Add more MultiFilereader features/hooks

[samansmink]

This PR adds some missing links for extending the MultiFileReader

Firstly I added a fieldcase_insensitive_map_t<Value> MultiFileReaderOptions::custom_options for passing custom options.

Secondly, I added the concept of a MultiFileReaderGlobalState. This is a state that should generally be created in the InitGlobal of a table function using the MultiFileReader. The global state allows the MultiFileReader to store state that is created while already knowing what columns are in the projection.

A crucial part of the MultiFileReaderGlobalState is the extra_columns param. This parameter will be set by the MultiFileReader to indicate that the scan will produce more columns than are actually projected. These columns are for internal use by the MultiFileReader during the FinalizeChunk step. This is crucial for the upcoming delta extension to be able to properly apply deletion vectors. To apply a deletion vector, we need to know which rows from the file are actually selected. This means the file_row_number column needs to be available in the FinalizeChunk step. However, this column should not be returned by the actual scan. The solution is very similar to what we currently do for Filter pruning: where columns that are only used for pushed down filters are removed during the scan.

@Mytherin I've managed to push most complexity into the delta extension for now to keep this PR simple, eventually we may want to pull the logic for populating the extra_columns up in the default MultiFileReader though

[Mytherin]

Thanks!