pimd: eBGP integration for SA loop detection

[rzalamena]

This PR implements the optional knob to enable eBGP integration to detect loops in SAs. The detection of MSDP SA loops works as follow:

1. eBGP AS number is configured in the MSDP peer
2. When SA is received the RP value is used in RPF check 2.a. If the RPF route AS Path number matches the neighbor AS then the SA is accepted 2.b. if the RPF route AS Path number doesn't match the neighbor (or doesn't exist) the SA is rejected (means that the SA is not coming from the shortest path)

[donaldsharp]

Is there any particular reason we've added a second slightly different address lookup for pim instead of just modifying the first one?

[github-actions[bot]]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[rzalamena]

Is there any particular reason we've added a second slightly different address lookup for pim instead of just modifying the first one?

Removed the new Zebra API and incremented the existing one (zebra next hop lookup). Now there is just a slight variation on the next hop lookup in PIM: there is a new function that does next hop lookup and also returns the BGP AS number.

[donaldsharp]

I do not see documentation for the new clear command? It would be nice that it was tested as well. Once that happens I'll get it in.

[rzalamena]

Rebased and added missing documentation for clear command. I don't think it is necessary to add a test for zeroing statistic counters.