This commit introduces a new thread pool, `system_read`, which is
intended for use by system indices for all read operations (get and
search). The `system_read` pool is a fixed thread pool with a maximum
number of threads equal to lesser of half of the available processors
or 5. Given the combination of both get and read operations in this
thread pool, the queue size has been set to 2000. The motivation for
this change is to allow system read operations to be serviced in spite
of the number of user searches.

In order to avoid a significant performance hit due to pattern matching
on all search requests, a new metadata flag is added to mark indices
as system or non-system. Previously created system indices will have
flag added to their metadata upon upgrade to a version with this
capability.

Additionally, this change also introduces a new class, `SystemIndices`,
which encapsulates logic around system indices. Currently, the class
provides a method to check if an index is a system index and a method
to find a matching index descriptor given the name of an index.

Relates elastic#50251
Relates elastic#37867
Backport of elastic#57936