LDP Protocol: Add support for downstream on demand configuration

[oscargdd]

Added support for downstream on demand in the LDP protocol configuration. LDP DoD enables on-demand label distribution ensuring that only required labels are requested, provided, and installed (see RFC 7032).

The proposed vendor-agnostic configuration and state for Downstream-on-demand is per neighbour. Hence, for each neighbour entry downstream-on-demand can be enabled. In the state information, the peer advertised label distribution model and final negotiated label distribution mode is added.

[OpenConfigBot]

Compatibility Report for commit b411496ab84e6f8c42ca4b8077c80bf88b468214: ⛔ yanglint@SO 1.10.17

[robshakir]

@oscargdd -- can you merge the two suggestions, which should re-run the CI, and then this LGTM.

Thanks!

[robshakir]

@oscargdd @sbarguil Looks like this one just needs merging of the master branch into it such that we can re-run the CI posted the pyangbind issue. I can then merge it.

[robshakir]

@dplore this is ready to merge, fyi.

folks -- please take a look and provide comments by 2022-08-08, at which point this will be merged.

[dplore]

/gcbrun

[dplore]

/gcbrun