Allow nested key pointers for target_index_key and target_type_key
Allow nested key pointers for target_index_key and target_type_key

Some docker plugins like Kubernetes or Docker metadata plugins adds nested objects that contains values usable for `target_type`. For example kubernetes `kubernetes.container-name` can be used for `target_type` value as different containers may have different log formats.
This patch allows to specify dot separated path for target_index_key and target_type_key to make this metadata plugins usable for this scenario.
It is backwards compatible with non nested keys. It only adds small additional overhead if used.
uken/fluent-plugin-elasticsearch,gtrevg/fluent-plugin-elasticsearch,cosmo0920/fluent-plugin-elasticsearch,aerickson/fluent-plugin-elasticsearch