Expose metadata and/or event index to codec

[bartelink]

At present, the OOTB codecs only present the data as UTF-8 to encode/tryDecode functions; need to extend to enable:

• generically wrapping events as an Envelope<T>
• combining UnionEncoder with grabbing context from the Event Data
• some degree of uniformity between consumption afforded by Equinox.Cosmos and Equinox.EventStore

Needs to consider #79 to some degree

🤔 Likely will involve introducing an IEvent into the codec module

cc @ameier38

[bartelink]

@eiriktsarpalis @ameier38 Merging this now as-is

Plumbing has been completed to make actually exposing Metadata and/or an Envelope surfacing curated metadata such an API based on (ideally >1) real world need:-

• IUnionEncoder gets passed the relevant information
• can downcast to the now-exposed IIndexedEvent to determine version numbers iff necessary
• low level Utf8BytesUnionEncoder is now exposed for people to be able to fork the canned ones