sstring: define formatter
sstring: define formatter

Formatting a basic_sstring compiles now, via decaying the sstring into
an std::string_view. However, it crashes. From my debugging it looks like
the temporary std::string_view is captured by reference, which is then
dangling. This happens only on aarch64, likely due to different optimization
choices by the compiler.

Fix by defining a formatter.

Closes #1249

scylladb/seastar,scylladb/seastar,scylladb/seastar