The definitions of virtual tables make up approximately a quarter of the
huge system_keyspace.cc file (almost 4K lines), pulling in a lot of
headers only used by them.
Move them to a separate source file to make system_keyspace.cc easier
for humans and compilers to digest.
This patch also moves the `register_virtual_tables()`,
`install_virtual_readers()` as well as the `virtual_tables` global.
Closes#14308
