Error reporting improvements

[adams85]

Describe the purpose of your pull request

Define error codes (EvaluationErrorCode, RefreshErrorCode) and expose them to the user (EvaluationDetails.ErrorCode, RefreshResult.ErrorCode) so they can reliably identify the error type without resorting to guess it by checking the error message (see also: https://github.com/open-feature/dotnet-sdk-contrib/pull/119#discussion_r1433928330)

The PR also proposes a new hook named ConfigFetched which could be used to observe config fetching-related errors even when refreshing is done by the SDK.

Related issues (only if applicable)

https://trello.com/c/0KrZmHgQ

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]]

Quality Gate failed

Failed conditions

67.8% Coverage on New Code (required ≥ 80%)

See analysis details on SonarCloud

[sonarcloud[bot]]

Quality Gate failed

Failed conditions
79.8% Coverage on New Code (required ≥ 80%)

See analysis details on SonarCloud