Gigya Web SDK integration - setup Flutter Web entrypoint

[navaronbracke]

This PR adds the web entrypoint for the example app.
The example web app won't run properly yet, since it currently lacks the most basic implementation of initSdk(), logout(), isLoggedIn(). Implementing that is done in #45

Part of #44

Changelog

Because we now have a web implementation part, I moved the method channel related stuff to its own folder

• moves method channel specific files into the src/method_channel folder (Android & iOS implementations)
• moves the various service interfaces to the src/services folder

Model class related changes

• adds the session cookie properties to the SessionInfo class
• Fix: Use followersCount instead of followersCounts when converting a Profile from JSON
• adds support for the data parameter in the Account class

BREAKING CHANGES

• Dart 3.0 and Flutter 3.10 are now the minimum supported version
  • This enables some features, such as switch expressions. As there have been several major releases (3.10, 3.13 and 3.16) I felt that it was the right time to bump the version constraint (also in preparation for the web version in the future)
• The SessionInfo class attributes are now nullable to account for differences between web and native
• Account.socialProviders is now a list of strings, which improves type safety
• Timestamps in the Account class have been refactored to become DateTime? instances.
  • The timestamp attributes have been renamed to their UTC string SAP SDK counterparts:
    • created, lastLogin, lastUpdated, oldestDataUpdated, registered, verified
    • signatureTimestamp has been updated to handle int, double and String for its parsing
  • The integer based timestamp fields have been removed, as the DateTime equivalents provide a better typing
• The startDate and endDate of a Certification is now a DateTime?, instead of String
• The timestamp field has been removed from the Like class. The time of a Like is now a DateTime
• The ConflictingAccounts class has been renamed to ConflictingAccount, the (web SDK) documentation uses singular for this object.

[navaronbracke]

Adding this constructor makes the yield implementation a bit cleaner, since we can just rely on a tear-off and let dart format format the call-chain for us

[navaronbracke]

Combining a stream transformer + a tear-off makes this read better.

[navaronbracke]

@tal-mi As the work for the Biometrics service has been merged into main now, I have made the following changes to this pull request:

• I have moved the biometrics_service.dart into the /services folder, to follow the same pattern as the other services
• I have moved the method_channel_biometrics_service.dart into the /method_channel folder, so that all the MethodChannel implementations are located in the same place.
• I ran dart format . and committed the formatting changes that were not applied yet.
• I ran dart analyze . and fixed one lint warning that was reported.

Once we get this pull request reviewed and merged, we can more easily split up the web implementation parts into smaller pieces. The PR's that I have opened for that, are currently drafts because most of their diffs are what is contained in this PR.