Allow defining routingtable.ClientOptions from ObserveRIBRequest

[netixx]

Describe the feature
I would like to be able to specify options of the routingtable.ClientOptions when I do my ObserveRIB request, such as:

• MaxPaths
• EcmpOnly
• BestOnly

This way the client can decide what type of paths it wants to receive.
Currently the options are hard-coded:

rib.RegisterWithOptions(rc, routingtable.ClientOptions{
		MaxPaths: 100,
	})

It would also be nice if we could specify those options for the DumpRIB request.

Use cases / additional context
I have a client that uses ObserveRIB to re-build a router RIB locally,
but I only need ECMP best paths, so I would like to be able to specify those options.

On other clients, we may want to get the full feed for observability purposes.

[taktv6]

I like the idea. Would definitely be willing to merge a PR that implements this. However, I won't find time to address this myself anytime soon.

[netixx]

Ok, here I go :)

I handle to ObserveRIB part here, for the DumpRIB, it would need more work.