pathd: Add optional support for PCEP to pathd

pathd: Add optional support for PCEP to pathd

This new dynamic module makes pathd behave as a PCC for dynamic candidate path
using the external library pcpelib https://github.com/volta-networks/pceplib .

The candidate paths defined as dynamic will trigger computation requests to the
configured PCE, and the PCE response will be used to update the policy.

It supports multiple PCE. The one with smaller precedence will be elected
as the master PCE, and only if the connection repeatedly fails, the PCC will
switch to another PCE.

Example of configuration:

segment-routing
 traffic-eng
  pcep
   pce-config CONF
    source-address ip 10.10.10.10
    sr-draft07
   !
   pce PCE1
    config CONF
    address ip 1.1.1.1
   !
   pce PCE2
    config CONF
    address ip 2.2.2.2
   !
   pcc
    peer PCE1 precedence 10
    peer PCE2 precedence 20
   !
  !
 !
!

Co-authored-by: Brady Johnson <brady@voltanet.io>
Co-authored-by: Emanuele Di Pascale <emanuele@voltanet.io>
Co-authored-by: GalaxyGorilla <sascha@netdef.org>
Co-authored-by: Javier Garcia <javier.garcia@voltanet.io>
Co-authored-by: Renato Westphal <renato@opensourcerouting.org>
Co-authored-by: Sebastien Merle <sebastien@netdef.org>
Signed-off-by: Sebastien Merle <sebastien@netdef.org>