Bug 4827: Consumer specific parsers for BGP add-path

Bug 4827: Consumer specific parsers for BGP add-path

The BGP add-path introduces extended form of NLRI (path-id added)
but new NLRI is not distiguishable from regular NLRI
(i.e. code points are the same).
The add-path capabilities are neighbor specific and information
about supported capabilities is available once the session with
a peer goes up. Also the BGP extension provides read-only
consumption of handlers registers, hence easiest way is to pass
the consumer-specific add-path constrain via input arguments.

Extend parser's registers to handle user defined constraints.
Added new NLRI registrator for add-path NLRI parsers.
Added PeerSpecificParserConstraint which can carry any future
user specific constraint to be used during parsing process.
Added MultiPathSupport interface to carry peer's specific
add-path capabilities.

Change-Id: Ieb361c726bd67f74fc653d3423870659404f1b9d
Signed-off-by: Milos Fabian <milfabia@cisco.com>