Trimming and Feature Flags

Feature flags are transformed by our adapter into keys looking like:

```
{[FeatureManagement:Alpha, False]}
```

The TrimKeyPrefix happens after this transformation. Meaning trimming any of the following would cause the feature flag to be unreadable (no longer in the expected "FeatureManagement" section):

```dotnet
.TrimKeyPrefix("f");
.TrimKeyPrefix("fe");
.TrimKeyPrefix("fea");
.TrimKeyPrefix("feat");
// etc
```

Trimming usually has some sort of divider which helps mitigate this- but I think trimming in it's current state and feature flags simply don't make sense to interact.

I propose we do not trim the key value if we know it is a feature flag. This is not a breaking change- as any trimming of a feature flag today would break it from being a feature flag. (You could trim a FF into acting like a normal KV by removing the full "FeatureManagement"- but I suspect little to no one has a dependency shaped like that)

@jimmyca15 @zhiyuanliang-ms @samsadsam