Fix to not create a watch to Internal.ServiceDump when mesh gateway is not used

[dhiaayachi]

Description

This add a fix to properly verify the gateway mode before creating a watch specific to mesh gateways. This watch have a high performance cost and when mesh gateways are not used is not used.

This also adds an optimization to only return the nodes when watching the Internal.ServiceDump RPC to avoid unnecessary disco chain compilation. As watches in proxy config only need the nodes.

Testing & Reproduction steps

Added a test to create a set of services with mesh gateway mode default and expecting that this watch is not effective.

Links

PR Checklist

• updated test coverage
• appropriate backport labels added
• not a security concern

[hashi-derek]

Excellent catch. Thank you for tracking this down and fixing it.

[hc-github-team-consul-core]

@dhiaayachi, a backport is missing for this PR [20168] for versions [1.15,1.16,1.17] please perform the backport manually and add the following snippet to your backport PR description:

<details>
	<summary> Overview of commits </summary>
		- <<backport commit 1>>
		- <<backport commit 2>>
		...
</details>