feat: introduce Codec entity

[weboko]

Problem / Description

For Send API that is part of Waku API to work we need a way to setup subscription when message is getting sent so that we could detect Filter and Store message loss.

For this Encoder is not sufficient.

Solution

We should introduce new entity instead of Encoder and Decoder that will combine their functionality and can be used in Filter and LightPush at the same time.

[adklempner]

Just realizing the naming conflicts, conceptually, with libp2p codecs. I'd prefer we name this entity something else.

[weboko]

closing as we agreed to go different direction in Send API