SearchTagRegistry: allow backing stores for tags that aren't std::vector

Currently, the default pattern for storing tags is to store them as
static base::NoDestructor<std::vector<SearchConcept>> instances.
However, this is actually unnecessary for the many search concepts, as
they have definitions that are fixed at compile time.

AddSearchTags() / RemoveSearchTags() now accept a span so callers can
use std::array or other backing stores for SearchConcepts.

Also simplify ProcessPendingSearchTags() simply use operator[]; the
inner loop is doing an insert-or-assign, which is exactly what
operator[] is intended for.

Bug: 375364966
Change-Id: Idc2d7229cd4baf3d19b67840eee545e5b52b8914
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5980264
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1377023}