Pim: Add support for PIM_MODE_BIDIR in openconfig-pim-types.yang

[raviv-arista]

The bidirectional pim mode is missing from the pim mode types.

    identity PIM_MODE_BIDIR {
        base PIM_MODE;
        description
          "PIM BIDIR mode.";
        reference "RFC5015";
    }
    

This should be added. I can open a PR for this.
Creating this issue to ask if there was a reason it wasn't added before.

[dplore]

Looks like it was only an oversight, happy to review a PR for this.