custom_calyptia: honour collector interval in secs and nano secs.

[niedbalski]

Description

This change makes the custom calyptia plugin honour the configured collector
interval (both in secs and nanosecs) that is cascaded to the instantiated input fleet plugin.

Also, I took the chance to add unit test to validate the cascade of properties to the underlying
fleet plugin.

Testing

• Example configuration file for the change

[SERVICE]
    log_level trace

[CUSTOM]
    Name calyptia
    API_Key xxxx.xxxx
    fleet_id e01085df-49d1-451d-b89f-226cd381369c
    Calyptia_Host cloud-api.calyptia.com
    fleet.interval_sec 120

• Debug log output from testing the change

---

Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

[edsiper]

@niedbalski Since the PR and commits touch different components, we need, as a minimum for this case 2 commits, one per plugin, ref: https://github.com/fluent/fluent-bit/blob/master/CONTRIBUTING.md#commit-changes . This is to have a clean history and log per component.

As soon as the commits are fixed we can merge it.

[cosmo0920]

Oh, I've overlooked that this PR touches the different components in the same commit.
We have to split commits and commit again with the same groups.
The changes should be reasonable. Just commit issue.