Add show command for BFD sessions

[shi-su]

What I did

Add show command for BFD sessions.

How I did it

Add show command for BFD session and unit tests.

How to verify it

Verify the show command locally and unit test passes.

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

Example output:

>> show bfd summary
Total number of BFD sessions: 3
Peer Addr    Interface    Vrf      State    Type          Local Addr      TX Interval    RX Interval    Multiplier  Multihop
-----------  -----------  -------  -------  ------------  ------------  -------------  -------------  ------------  ----------
10.0.1.1     default      default  DOWN     async_active  10.0.0.1                300            500             3  true
10.0.2.1     Ethernet12   default  UP       async_active  10.0.0.1                200            600             3  false
2000::10:1   default      default  UP       async_active  2000::1                 100            700             3  false

>> show bfd peer 10.0.1.1
Total number of BFD sessions for peer IP 10.0.1.1: 1
Peer Addr    Interface    Vrf      State    Type          Local Addr      TX Interval    RX Interval    Multiplier  Multihop
-----------  -----------  -------  -------  ------------  ------------  -------------  -------------  ------------  ----------
10.0.1.1     default      default  DOWN     async_active  10.0.0.1                300            500             3  true

[prsunny]

lgtm, please update the CLI command reference