before this change, we rely on the default-generated fmt::formatter created
from operator<<, but fmt v10 dropped the default-generated formatter.
in this change, we define formatters for internal types in service/storage_proxy.cc.
please note, `service::storage_proxy::remote::read_verb` is extracted out of
the outter class, because, the class's implementation formats `read_verb` in this
class. so we have to put the formatter at the place where its callers can see.
that's why it is moved up and out of `service::storage_proxy::remote`.
some of the operator<<:s are preserved, as they are still being used by
the existing formatters, for instance, the one for
`seastar::shared_ptr<>`, which is used to print
`seastar::shared_ptr<service::paxos_response_handler>`.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#17708