BGP: extract the bgp path attribute dissection to a dedicated function.

Change-Id: I228f1402007d8570bfdeb0105ce5e89c48357481
Reviewed-on: https://code.wireshark.org/review/5721
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
BGP: add support of RFC6368 - Internal BGP as the Provider/Customer Edge
     Protocol for BGP/MPLS IP Virtual Private Networks (VPNs).

Bug: 10762
Change-Id: Ifbe2af639ee5a2d96ec3546708060cadcc349df2
Reviewed-on: https://code.wireshark.org/review/5664
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>