Make content encoder and options explicitly typed and extensible

[kanongil]

This PR improves the typings of the route and payload compression options, by explicitly listing the available encoders (gzip & deflate) and types all the options.

Additionally, it allows users to register types for custom encoders/decoders with its own custom options. Eg.

import { createBrotliCompress } from 'zlib';

declare module '@hapi/hapi' {
    interface ContentEncoders {
        br: typeof createBrotliCompress;
    }
}

This will add and type a br encoder to servers.encoders(), and add it to the list of compression options with the expected typings.

Note that I deleted the nonsensical RouteCompressionEncoderSettings and PayloadCompressionDecoderSettings type exports (the settings have always been specific to each entry).