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, in order to enable the code in the header to
access the formatter without being moved down after the full specialization's
definition, we
* move the enum definition out of the class and before the
class,
* rename the enum's name from state to index_consume_entry_context_state
* define a formatter for index_consume_entry_context_state
* remove its operator<<().
as fmt v10 is able to use `format_as()` as a fallback, the formatter
full specialization is guarded with `#if FMT_VERSION < 10'00'00`. we
will remove it after we start build with fmt v10.
Refs #13245
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#16204
893f319004