Add support for compression dictionary transport

[pmeenan]

Compression dictionary transport is a HTTP mechanism for negotiating and using compression dictionaries for brotli and zstd-based content encodings. Depending on the use case, it can result in 90%+ smaller payloads than brotli or zstd alone.

It is also being defined in terms of fetch: whatwg/fetch#1854

For deno there are two interesting use cases:
1 - As defined in the spec where a dictionary is configured through the Use-As-Dictionary response header and used transparently for future requests.
2 - Applications may find it useful to explicitly configure a dictionary (that ships with the binary for example) to be used with a given fetch request. In this case, the same Available-Dictionary and Accept-Encoding can be used but it doesn't require that the dictionary be provided over the network.