login_client improvements

[Albert221]

After using this library in a production app, there were few issues discovered. This Pull Request fixes them.

Also, I went back to the onCredentialsChanged stream in the LoginClient class as it was in the 0.* versions of the login_client library. It's just much easier for the developer and cleaner to use stream instead of passing a callback that the developer would have to prospectively somehow reach from other parts of the code.

login_client

• OAuth2 clientId and clientSecret now default to an empty string.
• Export oauth2 Credentials, AuthorizationException and ExpirationException.
• Add onCredentialsChanged stream to LoginClient.
• Deprecate credentialsChangedCallback.

login_client_flutter

• Fix reading null credentials.

Closes #11.

[codecov]

Codecov Report

Merging #12 (ca18f96) into master (9a4a3ea) will decrease coverage by 27.18%.
The diff coverage is 54.54%.

@@             Coverage Diff             @@
##           master      #12       +/-   ##
===========================================
- Coverage   72.85%   45.66%   -27.19%     
===========================================
  Files           3       11        +8     
  Lines          70      127       +57     
===========================================
+ Hits           51       58        +7     
- Misses         19       69       +50     

Flag	Coverage Δ
cqrs	?
login_client	45.66% <54.54%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
packages/login_client/lib/src/oauth_settings.dart	33.33% <0.00%> (ø)
packages/login_client/lib/src/login_client.dart	76.27% <66.66%> (ø)
packages/cqrs/lib/src/cqrs.dart
packages/cqrs/lib/src/cqrs_exception.dart
packages/cqrs/lib/src/command_result.dart
...ages/login_client/lib/src/oauth2/custom_grant.dart	0.00% <0.00%> (ø)
...ckages/login_client/lib/src/refresh_exception.dart	66.66% <0.00%> (ø)
..._client/lib/src/strategies/sms_token_strategy.dart	0.00% <0.00%> (ø)
...t/lib/src/strategies/raw_credentials_strategy.dart	0.00% <0.00%> (ø)
... and 6 more

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