Support RFC4724

[digizeph]

RFC4724 describes the graceful restart mechanism of BGP routers. This includes a End-of-RIB marker as well as a BGP capability Graceful Restart Capability. The end-of-rib marker is used in BMP lifecycle.

• add function to recognize end-of-rib message (added in add support for end-of-RIB message test #138)
• add full open message capability parsing

Marker for End-of-RIB

https://datatracker.ietf.org/doc/html/rfc4724#section-2

An UPDATE message with no reachable Network Layer Reachability
Information (NLRI) and empty withdrawn NLRI is specified as the End-
of-RIB marker that can be used by a BGP speaker to indicate to its
peer the completion of the initial routing update after the session
is established.

Graceful Restart Capability

https://datatracker.ietf.org/doc/html/rfc4724#section-3

The Graceful Restart Capability is a new BGP capability [BGP-CAP]
that can be used by a BGP speaker to indicate its ability to preserve
its forwarding state during BGP restart. It can also be used to
convey to its peer its intention of generating the End-of-RIB marker
upon the completion of its initial routing updates.