Expose Request "config" and "transformers" fields in transform_wasm

[fstanis]

trafficstars

This would help us with integrating into other projects, such as amp-toolbox.

Proposed valid invocations:

transform('https://example.com/', html) // uses DEFAULT config

transform('https://example.com/', {
  config: 'VALIDATION'
})

transform('https://example.com/', html, {
  config: 'CUSTOM',
  transformers: [
    'ampboilerplate',
    'urlrewrite'
  ]
})

[twifkak]

Good idea. Not necessary for signed exchanges, so apologies for the late milestone, but it should be an easy fix, so I'll likely get to it sooner (and am happy to accept PRs as long as they don't introduce memory leaks).