feat(aws-lambda) separate aws credential cache in aws-lambda plugin by plugin identifier in config

[windmgc]

Summary

This PR is trying to separate AWS credential cache keys which are used by different aws-lambda plugins, by adding a plugin identifier in the mlcache key name.

Users might want to enable multiple aws-lambda plugins, targeting at different IAM roles and different lambda functions. Under this situation credential cache must be separated to avoid mutual overwrite

Full changelog

• Adding conf.aws_assume_role_arn as plugin identifier to the credential cache key name

Issue reference

No issue reference for now.

[windmgc]

I wonder if there is a more proper/elegant way of adding plugin identifiers into the cache key instead of just using config.__key__.(it seems quite long, and the variable name have underscores which make it seems more like an internal variable)
I'd be grateful if anyone have good ideas on that

[windmgc]

Decide to use the target IAM role ARN as the cache key identifier because the credential will only change when the target role changes. Using conf.__key__ will make cache keys way more distributed(per service/route/consumer).