refactor(dart): update the token storage to store a map of access token instance

[simeng-li]

Summary

This PR includes the following updates:

1. Create a new AccessToken interface that includes token, scope, and expiresAt props.
2. Refactor the TokenStorage access token storage type as a map of the AccessToken instance, using scope + resource as their keys. In order to support multiple resource audience access token storage.
3. The AccessToken map data will be serialized into a string for FlutterSecureStorage.
4. Reorg the utility folder, and extract some of the classes as modules, for better management.

Testing

UT case updated
@logto-io/eng @logto-io/mandatory-reviewers cc: @julian-hartl @JulianHartl

[linear]

LOG-4263 Update the access_token storage