Backport of Add validation for apigw creation with no routes into release/1.15.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #16649 to be assessed for backporting due to the inclusion of the label backport/1.15.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

merge conflict error: unable to process merge commit: "25828df8bfa72177758b4289257d8ccade3f2bc6", automatic backport requires rebase workflow

The below text is copied from the body of the original PR.

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Description

To meet the standard (Gateways require at least one listener defined) we need to add checks in to verify that at least one listener is defined.

Testing & Reproduction steps

make dev-build 
consul agent -dev

Open another terminal, create a file called gateway.hcl with the following content:

Kind = "api-gateway"
Name = "my-gateway"

After creating the file, run the following command in the same terminal:

consul config write gateway.hcl

This should error out, saying "api gateway must have at least one listener"

PR Checklist

• updated test coverage
• external facing docs updated
• not a security concern

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Overview of commits

• b22ec0d - 19534a7 - 65b82cb - 25828df - aa428e9 - d06e891 - 893851c - 75cdf06 - 8956953 - 76124d0 - c0de303 - 3831701 - a1a1a27 - 812bcf8

[github-team-consul-core-pr-approver]

Auto approved Consul Bot automated PR