Added UDP IPV4 Encapsulation Header as per requirement

[keysightgems]

The changes were required based on the README/PR below, so I added the IPv4 UDP encapsulation header code accordingly. Please review and merge the changes.

Readme Location: https://github.com/openconfig/featureprofiles/blob/main/feature/gribi/mpls_in_udp/otg_tests/mpls_in_udp_scale/README.md
Raised PR for the Readme: openconfig/featureprofiles#5714

[robshakir]

Thanks for the change.

Please can you also add a test into fluent_test.go's TestEntry table-driven test that validates that this functionality works as expected within an entry?

[keysightgems]

Thanks for the change.

Please can you also add a test into fluent_test.go's TestEntry table-driven test that validates that this functionality works as expected within an entry?

Addressed all the comments, validated the changes, and committed the code.

ixia@ubserver-2-2404:~/Phase2/Giribigo/gribigo-copy$ go test ./fluent/ -run TestEntry -v
=== RUN TestEntry
=== RUN TestEntry/prefix_populated_only
=== RUN TestEntry/prefix,_nhg,_ni
=== RUN TestEntry/next-hop-group
=== RUN TestEntry/next-hop
=== RUN TestEntry/next-hop_interface_only
=== RUN TestEntry/nhg_with_a_backup
=== RUN TestEntry/next-hop_with_decap
=== RUN TestEntry/mpls_entry
=== RUN TestEntry/ipv6_entry
=== RUN TestEntry/next-hop_mpls-over-udpv6
=== RUN TestEntry/next-hop_mpls-over-udpv4
--- PASS: TestEntry (0.02s)
--- PASS: TestEntry/prefix_populated_only (0.00s)
--- PASS: TestEntry/prefix,_nhg,_ni (0.00s)
--- PASS: TestEntry/next-hop-group (0.00s)
--- PASS: TestEntry/next-hop (0.00s)
--- PASS: TestEntry/next-hop_interface_only (0.00s)
--- PASS: TestEntry/nhg_with_a_backup (0.00s)
--- PASS: TestEntry/next-hop_with_decap (0.00s)
--- PASS: TestEntry/mpls_entry (0.00s)
--- PASS: TestEntry/ipv6_entry (0.00s)
--- PASS: TestEntry/next-hop_mpls-over-udpv6 (0.00s)
--- PASS: TestEntry/next-hop_mpls-over-udpv4 (0.00s)
PASS
ok github.com/openconfig/gribigo/fluent 0.258s

[keysightgems]

@robshakir

Thanks for merging the PR.
Can you please create a new version of gribigo to get the changes to FP.