Move AMP runtime to standard fetch

[prateekbh]

This issue serves as a tracker to move AMP to standard fetch instead of the used poly/ponyfills of fetch, FetchResponse and FetchResponseHeaders.

The above methods/classes are always used instead of native fetch if we want to request a document over the network. This effort will isolate this case in a separate ES-module and move all the rest of xhr-impl.js over to native fetch whenever present.

The entire migration will be done in four tasks.

• Create a separate DocumentFetcher ES module and implement fetchDocument.
• Shift current definitions of fetch, FetchResponse and FetchResponseHeaders to a fetch-polyfill instead.
• Move all existing usage to document fetcher and remove all the code for fetchDocument form xhr-impl.js.
• Rename Services.xhr to Services.fetch.
• Decide on a uniform method to use .json(), .text() etc methods from fetch and create eslint rules to confirm it throughout the codebase.

[ampprojectbot]

This issue seems to be in Pending Triage for awhile. @prateekbh Please triage this to an appropriate milestone.

[prateekbh]

Fixed by #17642