Add a metadata parser

[joachimvh]

Resolves #75 .

Since we now have decent metadata support, we still need a clean way to put the incoming metadata in there. So this is my suggestion for it.

Note that for the MetadataParsers their functionality is similar to AsyncHandlers but I used a new interface since the contracts is slightly different (it's no problem if they can't handle it, and all parsers should be executed regardless of the result of others).

[RubenVerborgh]

I like the handler idea, but

      metadata: await this.metadataHandler.handleSafe(input),

seems repetitive.

Why don't the parsers just not return any metadata?
And then we have one wrapping parser whose sole job it is to add the metadata (so to execute that line)?

[joachimvh]

Why don't the parsers just not return any metadata?
And then we have one wrapping parser whose sole job it is to add the metadata (so to execute that line)?

The reason it's currently like this is because BodyParsers return a Representation which needs to contain a RepresentationMetadata. Other options include replacing that line for every BodyParser with metadata: new RepresentationMetadata(), and then passing that metadata object along to the MetadataHandler (is that what you meant with the above statement?). Or running the MetadataHandler first and then passing the resulting metadata to the BodyParsers. Or making metadata optional in the RepresentationMetadata (maybe not this one since several parts of the code assume there is always a content-type so would require several extra checks further down the line).

[RubenVerborgh]

(After offline discussion:)

Let's have body parsers take metadata as input, to which they can add if they want to, and then return as part of the representation. The metadata is provided by metadata extractors, and can inform body parsing.

[RubenVerborgh]

Really good stuff, I like the design. Very elegant an extensible.

Just nits—process as you see fit. No need for me to re-review.