Support interoperability with other version of itself

This would solve...

undici is not compatible with other versions of itself, or Node's.

If a library uses undici's fetch() internally and the user of the library uses Node's Request, or vice versa, fetch() call will fail with an error like so:

  TypeError: Failed to parse URL from [object Request]
      at fetch (~/my-lib/node_modules/.pnpm/undici@6.11.1/node_modules/undici/index.js:109:13) {
    [cause]: TypeError: Invalid URL
        at new URL (node:internal/url:804:36)
        at new Request (~/my-lib/node_modules/.pnpm/undici@6.11.1/node_modules/undici/lib/web/fetch/request.js:88:21)
        at fetch (~/my-lib/node_modules/.pnpm/undici@6.11.1/node_modules/undici/lib/web/fetch/index.js:136:21)
        at fetch (~/my-lib/node_modules/.pnpm/undici@6.11.1/node_modules/undici/index.js:106:18)
        at fetch (~/my-lib/packages/pds/dist/auth-provider.js:42:51)
        at fetchTimeout (~/my-lib/packages/fetch/dist/fetch-wrap.js:61:28)
        at ~/my-lib/packages/fetch/dist/fetch-wrap.js:39:29
        at ~/my-lib/packages/transformer/dist/compose.js:20:47
        at async i (~/my-lib/packages/transformer/dist/compose.js:16:51)
        at async CachedGetter.get (~/my-lib/packages/caching/dist/cached-getter.js:97:20) {
      code: 'ERR_INVALID_URL',
      input: '[object Request]'
    }

This error is misleading as the input is actually a Request that gets casted to string because it comes from another version of undici:

https://github.com/nodejs/undici/blob/f51f226522ec75a0613a07a4efc8f78938030c45/lib/web/fetch/request.js#L924-L934

The implementation should look like...

When checking if an input is a request, instead of using instanceof, an thorough interface check could be performed.
Every fields from the input should be checked against the spec's interface https://fetch.spec.whatwg.org/#requestinfo

webidl.converters.RequestInfo = function (V) {
  if (typeof V === 'string') {
    return webidl.converters.USVString(V)
  }

  if (V instanceof Request) {
    return webidl.converters.Request(V)
  }

  const str = webidl.converters.USVString(V)
  if (str === '[object Request]') {
    // Request from another version of undici (or node's internal undici)
    return webidl.converters.RequestInterface(V) // <== TODO implement this
  }

  return src
}

Note that every field of the input Request (including dispatcher) should be carried over to the new request.

Symbols would probably need to be using a globally namespaced form instead of locally declared symbols (Symbol.for('undici.XYZ')) for the initalization process to work here:

https://github.com/nodejs/undici/blob/f51f226522ec75a0613a07a4efc8f78938030c45/lib/web/fetch/request.js#L107-L118

I have also considered...

Additional context

I am trying to make a library to ease the use of fetch() by allowing to tranform Requests, and wrap node's fetch(). This is not possible if Request/fetch from different sources are mixed together

Start in lib/web/fetch/request.js at the RequestInfo conversion and request initialization areas referenced by the issue. Read the Fetch RequestInfo specification and trace how cross-version Request objects and symbols are recognized. Done means requests from other undici versions or Node's implementation work with fetch while preserving all input fields, including dispatcher.