ref-filter.h: move contains caches into filter

ref-filter.h: move contains caches into filter

Move the 'contains_cache' and 'no_contains_cache' used in filter_refs into
an 'internal' struct of the 'struct ref_filter'. In later patches, the
'struct ref_filter *' will be a common data structure across multiple
filtering functions. These caches are part of the common functionality the
filter struct will support, so they are updated to be internally accessible
wherever the filter is used.

The design used here mirrors what was introduced in 576de3d956
(unpack_trees: start splitting internal fields from public API, 2023-02-27)
for 'unpack_trees_options'.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>