feat: Add config option to filter events based on namespace

[MikeGoldsmith]

Which problem is this PR solving?

When deploying the agent in an environment where some apps may already be instrumented and the agent would be duplicating events. It is desireable that some namespaces be omitted from generating events.

This PR adds a configuration option to provide a list of namespaces that will be used as an exclude list. When either the source or destination IP matches a pod or service that is in an excluded namespaces, it is not converted into a libhoney event.

• Closes Configurable capture filter based on k8s pod and/or namespace? #188

Short description of the changes

• Add config option to hold lookup map of namespaces that is populated from the NAMESPACES env var built from a comma separated list
• Extend the liboney event handler to check whether it should send the event by looking up the asscociated resource namespace
• Update README with new config option
• Add unit tests for config and libhoney handler

How to verify that this has the expected result

It's now possible to provide a list of namespaces that will be used ignore events when the event's source or destination resource is in that namespace.

[robbkidd]

@MikeGoldsmith and I were talking about this out-of-band. We suspect we don't yet know in what specific cases users would want to omit traffic to/from a namespace.

Should the filter only exclude traffic where both the source and destination are k8s resources within excluded namespaces?

[MikeGoldsmith]

Closing while we work out what traffic we want to omit when a namespace matches. This could be re-opened or a new one using this as the basis.