Add support for logaggregation to a path on non-default filecontext

Current log-aggregation implementation supports logaggregation to default filecontext only. This patch is to support logaggregation to any of the supported filesystems within hadoop eco-system (hdfs, s3, swiftfs etc). So for example a customer could use hdfs as default filesystem but use s3 or swiftfs for logaggregation. Current implementation makes mixed usages of FileContext+AbstractFileSystem apis as well as FileSystem apis which is confusing.

This patch does two things:

1. moves logaggregation implementation to use only FileContext apis
2. adds support for doing log aggregation on non-default filesystem as well.
3. changes TestLogAggregationService to use local filesystem itself instead of mocking the behavior