Add Filtering support

[edsiper]

Fluent Bit needs to get filtering plugins support, so data form inputs can be modified.

First priority is to get a Kubernetes metadata filter.

[mitchhh22]

+1 for Kubernetes metadata filter

[nokute78]

Note: Some input plugins are still work in progress to support filter. See also #173

[edsiper]

Filtering support is already in place, current plugins are:

• stdout
• grep

The branch filter_kubernetes have the progress of that new upcoming filter.