Offline mode feature

[adams85]

Describe the purpose of your pull request

• Adds IConfigCatClient.IsOffline property, IConfigCatClient.SetOnline and IConfigCatClient.SetOffline methods.
• Renames EmptyConfigService to NullConfigService for better naming as this class implements the null object design pattern. (This is just a suggestion though, we can scrap it if you disagree.)

It's also worth mentioning that in the current implementation the client goes into offline mode implicitly when gets disposed. This also prevents the user from making forced cache refreshes (RefreshConfig/RefreshConfigAsync) on a disposed client. If you don't think that this is a good design, let's discuss it.

Requirement checklist (only if applicable)

• [x] I have covered the applied changes with automated tests.
• [x] I have executed the full automated test set against my changes.
• [x] I have validated my changes against all supported platform versions.
• [x] I have read and accepted the contribution agreement.

[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

92.5% Coverage
2.2% Duplication