crimson/osd: add tell command support

crimson/osd: add tell command support

* change the function signature of `call()` to accept a bufferlist,
  as MCommand is supposed to carry a bufferlist which can be consumed
  by the hook.
* drop the wrapper of `tell()`, instead always use `call()`. simpler
  this way.
* promote `tell_result_t` to top level of `crimson::admin` as it is used
  as part of the interface between hook and `AdminSocket`
* replace `maybe_parsed_t` with `std::variant<parsed_command_t, tell_result_t>`,
  as the latter is able to convey more detailed error information if
  something goes wrong.
* osd now handles `MCommand`.

Signed-off-by: Kefu Chai <kchai@redhat.com>