Consider improving LDUser::getValueForEvaluation performance for custom attributes

[rbalamohan]

For custom attributes, currently {{LDUSer::getValueForEvaluation}} checks for {{UserAttribute}} first and moves on to check "custom" attributes.

https://github.com/launchdarkly/java-client/blob/8b29441d5870ecc8b20d31d0b2fff2c5a32007de/src/main/java/com/launchdarkly/client/LDUser.java#L73

https://github.com/launchdarkly/java-client/blob/8b29441d5870ecc8b20d31d0b2fff2c5a32007de/src/main/java/com/launchdarkly/client/LDUser.java#L77

As a part of this, it always encounters "IllegalArguementException" which ends up populating the entire stacktrace (which could be 100s in size). This turns out to be very expensive depending on the number of calls.

It would be good consider, where custom attributes are checked first and then check the user attribute. This way, we do not need to pay the penalty of filling up the stack trace for every call involving custom attribute.

[eli-darkly]

See PR response. There should be a fix for this soon. Thanks for pointing it out.

[eli-darkly]

This has been fixed in releases 2.6.1 and 3.0.2.