[Feature]: filebeat inputs

Currently, the Filebeat role requires adjustments whenever a new input is added. I suggest leaving the configuration of Filebeat inputs to the user instead.

This would make the role more dynamic and flexible:

• Users can define their own key:value pairs directly in the configuration file.

• They take responsibility for the correctness of the content.

• We avoid the overhead of maintaining Jinja code for ~33 different Filebeat inputs.

• It also reduces the need to modify the template every time a new input type is requested.

I don’t yet have a 100% concrete idea of what the final template should look like, but the approach should be feasible.

What do you think @widhalmt

Design note (see #505/#510): same declarative approach — one list = desired state, rendered inline. This subsumes beats_fields and the docker/log/syslog input blocks.