Use msgpack extension types instead of custom swh encoders/decoders

(At least in the context of the journal) We currently use custom encoders for a few object types when serializing swh objects in msgpack: we currently use the encoding schema:

{
 b'swhtype': '<type>',
 b'd': <payload>
}

while the msgpack format allows custom [[ https://github.com/msgpack/msgpack/blob/master/spec.md#extension-types | extension types ]] (which has recently been used to prevent integer overflows in msgpack dump/load).

We should make use of these extension types instead, it would make more sense, be more consistent and make the documentation of the kafka messages (swh-docs#2818 (closed)) easier.

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Migrated from T2834 (view on Phabricator)