feat(fluentd): Input plugin CRs

[MisterMX]

What this PR does / why we need it:

This adds two new CRs Input and ClusterInput to fluentd that are collected like Output and ClusterOutput. They allow specifying Fluentd input plugins individually as an alternative to spec.globalInputs in the FluentD resource.

It also adds the ability to define custom plugins in the input.Input struct.

---
apiVersion: fluentd.fluent.io/v1alpha1
kind: ClusterInput
metadata:
  name: cluster-fluentd-input-cw
  labels:
    input.fluentd.fluent.io/scope: "cluster"
    input.fluentd.fluent.io/enabled: "true"
spec:
  inputs:
    - customPlugin:
        config: |
          <source>
            @type cloudwatch_logs
            tag cloudwatch.in.kafka
            include_metadata true
            log_group_name /aws/kafka/%s
            log_stream_name %s
            use_log_group_name_prefix true
            use_log_stream_name_prefix true
            region {{ $region }}
            use_aws_timestamp true
            format none
            throttling_retry_seconds %s
            <storage>
              @type local
              path /var/log/fluent/cloudwatch.in.kafka.state
            </storage>
          </source>

Support custom plugins in input.Input struct.

Which issue(s) this PR fixes:

Fixes #971

Does this PR introduced a user-facing change?

It adds two new structs but no breaking changes.

Additional documentation, usage docs, etc.:

[benjaminhuo]

Thanks @MisterMX for this great feature. Would you help to tix the lint-test? https://github.com/fluent/fluent-operator/actions/runs/6694949950/job/18290100007?pr=972

You may also need to run make docs-update to generate docs

[MisterMX]

I bumped helm/chart-testing-action to cb49023b9227b1097e5eddd8824f48bdea11b1aa due to helm/chart-testing-action#132 (comment).

[benjaminhuo]

I bumped helm/chart-testing-action to cb49023b9227b1097e5eddd8824f48bdea11b1aa due to helm/chart-testing-action#132 (comment).

Great! Thanks!