[os-frr] Add multihop support to BFD web interface

[KellenRenshaw]

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Is your feature request related to a problem? Please describe.
I am running BGP on loopback addresses and would like to use BFD between the loopback addresses. This requires the "multihop" option on the peer directive in BFD. This was mentioned in #2050 but I don't think it made it into the merge.

Describe the solution you'd like
I would like a checkbox in the "Edit Neighbor" modal of the Neighbors tab at /ui/quagga/bfd/index that says "Enable Multihop" or something to that effect.

This checkbox should cause the configuration for the peer to be written as:
peer X.X.X.X multihop
instead of:
peer X.X.X.X

Describe alternatives you've considered
I could manually edit the configuration to make it work, but exposing the option in the UI is cleaner and prevents accidental overwrites on save or reboot.