Add Destination Field to Service-Defaults for Tproxy Egress

[DanStough]

Changes proposed in this PR:

• Adds the "Destination" field to the "ServiceDefaults" CRD spec to be consistent with the consul core API. This enables external mesh traffic to be routed through terminating gateways in transparent mode.
• Minor CONTRIBUTING update based on first-time contributor experience.

How I've tested this PR:

Short version (detailed test plan available upon request)

• Build control-plane
• Deploy through Helm chart to local cluster w/ transparent proxy enabled and dev image override.
• Add static-client application
• Add service-default CRD w/ destination to "httpbin.org:443"
• Add mesh CRD with "meshDestinationsOnly" enabled
• Add terminating gateway CRD w/ link to "httpbin" service default
• Exec into client pod and "curl https://httpbin.org/uuid"

Acceptance Tests(?): TBD

How I expect reviewers to test this PR:

• Something similar to the above process.

Checklist:
TBD Below

• Tests added
• CHANGELOG entry added

[DanStough]

Looks like there is some weirdness w/ the new SDK update. Will investigate.

[thisisnotashwin]

This is excellent!! Have a comment that should be straightforward to address, but excellent work!!

[ishustava]

Great work, Dan!! This looks fantastic! Just a few minor comments.