Implement non-blocking synchronous evaluation (snapshot API)

[adams85]

Describe the purpose of your pull request

Adds an API which makes it possible to synchronously evaluate feature flags/settings without block waiting for potential underlying I/O operations: consumers can create a snapshot of the client by IConfigCatClient.Snapshot(), which captures the client's state (including the latest config fetched) at that point, then, using the returned object, they can execute non-blocking synchronous evaluation operations.

Also

• adds a parameter named CacheState to the event args of the ClientReady hook, by means of which consumers can get information about the initialization state of the client,
• adds warnings to the XML documentation of the blocking synchronous methods of IConfigCatClient.

Requirement checklist (only if applicable)

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

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
10 Code Smells

85.8% Coverage
0.2% Duplication

[sonarcloud]

Quality Gate passed

The SonarCloud Quality Gate passed, but some issues were introduced.

10 New issues
0 Security Hotspots
85.4% Coverage on New Code
0.2% Duplication on New Code

See analysis details on SonarCloud

[sonarcloud]

Quality Gate passed

Issues
23 New issues
0 Accepted issues

Measures
0 Security Hotspots
86.8% Coverage on New Code
0.2% Duplication on New Code

See analysis details on SonarCloud