validate property types of feature flags

[Eskibear]

See #14
It validates property types based on the feature flag JSON schema.

[zhiyuanliang-ms]

This PR will not close #14. After we support telemetry and variant, we need to validate their properties.

[zhiyuanliang-ms]

@Eskibear @linglingye001 I am thinking whether the validation is too much. In our current implementation, the validation will happen during each call of isEnabled, this could be a performance bottle neck.

In .NET FM, the validation will only happen once, because feature flag provider has a cache mechanism (but we don't have that and I am also wondering whether it is possible to have that because we cannot know whether the configuration changed so that we can refresh the cache.)

So, I am thinking maybe we can add an option to disable the feature flag validation. As long as the feature flag is from app config, we should have the confidence that feature flags are in the correct shape.

[zhiyuanliang-ms]

Except the comments I left, this PR looks good to me.

[Eskibear]

@zhiyuanliang-ms do you want me to update the PR or you take it over?

[zhiyuanliang-ms]

@zhiyuanliang-ms do you want me to update the PR or you take it over?

@Eskibear I asked @linglingye001 to take over this PR. Did you notice that lingling has pushed a lot of commits?