Support custom matching attributes in CentralizedRule

Java SDK: `com.amazonaws:aws-xray-recorder-sdk-aws-sdk-v2:2.4.0`

I just spent a good amount of time figuring out why the SDK told me that my sampling rule (created in the official AWS console) is "invalid".

Turns out it is because custom attribute matching is not supported, see https://github.com/aws/aws-xray-sdk-java/blob/126b60ea543509f9cd6510b5dff833947d826d78/aws-xray-recorder-sdk-core/src/main/java/com/amazonaws/xray/strategy/sampling/rule/CentralizedRule.java#L105.

It would be really nice if this was supported, but if not at least the error message should be more helpful. This is an official feature as it is available in the X-Ray console, so it's a bit weird to get told that the rule created with the official tools is invalid.

In this case I wanted to limit X-Ray sampling from one specific IP.