new: Added support for configuration compatible with ECS log format

[pamburus]

• Added support for predefined field names that can match fields in nested objects when used with dot delimiters.
  For example, if log.origin.file.name is used for a predefined field name, it will match any of the
  • { "log.origin.file.name": "app.py" }
  • { "log": { "origin.file.name": "app.py" } }
  • { "log": { "origin": {"file.name": "app.py" } } }
  • { "log": { "origin": {"file": {"name": "app.py" } } } }
  • etc.
• Added predefined caller-file and caller-line fields for separate processing of file name and line number.

Example output

23-11-02 08:09:10.423 |INF| __main__: backup for database bar in foo is already done today application='backup-rds' ecs={ version='1.6.0' } language='python' process={ name='MainProcess' pid=1 thread={ id=140610736142144 name='MainThread' } } ... @ app.py:107

Suggested configuration file for ECS logs is etc/defaults/config-ecs.yaml
These settings have not been added to the default configuration because it is not yet clear how they will affect performance.

Closes #97