PRODUCT-775: support for extensions and PRF on Android, iOS and Windows

[keithbucher]

This PR adds support for extensions to our fork of the flutter-passkeys repository. Simple key-value extensions should work. Extensions that require additional parsing may not. However, the driving purpose of this PR is adding support for prf so that we can use it for E2EE from the Dapple Authenticator application. It is based on this PR in the Corbado repository. But that PR is limited to just the prf extension and overall not well-written IMO.

Notes:

• This has been tested on Android, iOS and Windows
• evalByCredential has not been tested
  EDIT:
• JSON parsing is currently done by custom code on Windows since all the available libraries seemed heavyweight or Chinese controlled. Our parsing needs are fairly minimal, so I think this should be ok
• JSON parsing has been moved to this JSON parser, which seems to fit our minimal needs and be maintained and used by US-based companies

To test with our authenticator-app you'll need the following in your pubspec.yaml (and flutter-passkeys checked out alongside authenticator-app)

dependencies:
...
  passkeys:
    path: ../../flutter-passkeys/packages/passkeys/passkeys
...
dependency_overrides:
...
  passkeys_android:
    path: ../../flutter-passkeys/packages/passkeys/passkeys_android
  passkeys_darwin:
    path: ../../flutter-passkeys/packages/passkeys/passkeys_darwin
  passkeys_platform_interface:
    path: ../../flutter-passkeys/packages/passkeys/passkeys_platform_interface
  passkeys_web:
    path: ../../flutter-passkeys/packages/passkeys/passkeys_web
  passkeys_windows:
    path: ../../flutter-passkeys/packages/passkeys/passkeys_windows
  passkeys_doctor:
    path: ../../flutter-passkeys/packages/passkeys/passkeys_doctor

[disptr]

Looks good!

Dropped a couple of comments with nice-to-haves (although I am worried that the JSON parser might break for certain data).