infra for snmp support and linkUp/Down and Config change traps

[suresh-rupanagudi]

- What I did
Infra for snmp trap generation and support for linkUp/Down and Config change traps.
- How I did it
Trap infra provides a way to applications to register db keys changes interested and callback function that needs to be called on
db change.
- How to verify it
1)Configure snmp trap receiver
2)shut/noshut interface

- Description for the changelog

UT:
1)
config snmptrap modify 2 <trap_receiver_ip>
config vlan add 100

trap revived at receiver:
11:29:04.246743 IP (tos 0x0, ttl 64, id 18847, offset 0, flags [DF], proto UDP (17), length 103)
10.59.143.181.55851 > 10.59.142.44.162: [udp sum ok] { SNMPv2c { V2Trap(60) R=647415881 .1.3.6.1.2.1.1.3.0=13371 .1.3.6.1.6.3.1.1.4.1.0=.1.3.6.1.4.1.441
3.1.2.2.1.2.0.1 } }

config interface shutdown Ethernet0

Trap received at receiver:
11:33:20.007605 IP (tos 0x0, ttl 64, id 53955, offset 0, flags [DF], proto UDP (17), length 178)
10.59.143.181.55851 > 10.59.142.44.162: [udp sum ok] { SNMPv2c { V2Trap(133) R=647417338 .1.3.6.1.2.1.1.3.0=38947 .1.3.6.1.6.3.1.1.4.1.0=.1.3.6.1.6.3.1.
1.5.3 .1.3.6.1.2.1.2.2.1.1.1=1 .1.3.6.1.2.1.2.2.1.7.1=2 .1.3.6.1.2.1.2.2.1.8.1=2 .1.3.6.1.6.3.1.1.4.3.0=.1.3.6.1.4.1.8072.3.2.10 } }

[suresh-rupanagudi]

sonic-net/sonic-buildimage#8130
this pull request also required.

[lgtm-com]

This pull request introduces 4 alerts when merging 7195611 into 7a78703 - view on LGTM.com

new alerts:

• 4 for Unused import

[lgtm-com]

This pull request introduces 4 alerts when merging defbf17 into 2654f4a - view on LGTM.com

new alerts:

• 4 for Unused import

[praveen-ramamoorthi]

We require this change to support SNMP Link up and Link down Trap messages. Kindly let me know if any other change is required in this PR, as it's in review for a long time.