Allow specifying the object key in the `aws_s3` sink

Requested by user migrating from fluentd.

Currently, we expose some configuration knobs for the object prefix + key for the aws_s3 sink:

  filename_append_uuid = true # optional, default
  filename_extension = "log" # optional, default
  filename_time_format = "%s" # optional, default
  key_prefix = "date=%F/" # optional, default

However there is no way to directly set the key. Ideally this could be templated from the event (like key = "{{ host }}"). This would affect internal partitioning of events across S3 requests. It is likely we'll still want to support formatting the timestamp as part of that, as well.

Ref: fluentd's s3 output: https://docs.fluentd.org/output/s3#s-3-_object_key_format