cmake: add an option "WITH_FMT_HEADER_ONLY"

cmake: add an option "WITH_FMT_HEADER_ONLY"

in this change:

* an interface library named "fmt-header-only" is introduced. it brings
  the support to the header only fmt library.
* fmt::fmt is renamed to fmt
* an option named "WITH_FMT_HEADER_ONLY" is introduced
* fmt::fmt is an alias of "fmt-header-only" if "WITH_FMT_HEADER_ONLY"
  is "ON", and an alias of "fmt" otherwise.

because fmt is packaged in EPEL, while librados is packaged
in RHEL, so we cannot have fmt as a runtime dependency of librados.
to address this issue an option "WITH_FMT_HEADER_ONLY" is introduced, so
that we can enable it when building Ceph with the header version of fmt.
and the built packages won't have runtime dependency of fmt.

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