common: add versioned encodings for std::variant

common: add versioned encodings for std::variant

adds two encoding strategies for `std::variant<>` under the namespaces
`ceph::versioned_variant` and `ceph::converted_variant`

these versioned encodings allow the variant to be extended with new
types, provided that they're always added to the end without changing
or removing existing types. because of this requirement, no default
encoding is provided for `std::variant`. callers must opt in to one
namespace or the other

the `converted_variant` encoding requires the variant's first type T
to use versioned encoding, and guarantees that the variant's encoding
is backward-compatible with T's

Signed-off-by: Casey Bodley <cbodley@redhat.com>