lib: Add CSPF Path Computation algorithm

lib: Add CSPF Path Computation algorithm

As helper function of Segment Routing Flex Algo or RSVP-TE
add Constrained Shortest Path First algorithm able to compute
path with constraints. Supported constraints are as follow:
 - Standard IGP metric
 - TE IGP metric
 - Delay metric
 - Bandwidth for given Class of Service for bandwidth reservation (RSVP-TE)

Usage of CSPF algorithms is detailed in the doc/developer/cspf.rst file

Signed-off-by: Olivier Dugeon <olivier.dugeon@orange.com>