tcp filter: Allow referencing dynamic clusters in v1 config

[snowp]

This removes the validation code that ensures that any cluster referenced in the tcp filter config is known when the config is applied. Removing this allows users to specify a cluster that is not known at the time but will be made available later on through CDS.

Risk Level: Low
This is already possible to do with the v2 config (as the validation was not applied in that case), so the behavior should already be battle tested. This does open up the possibility for people misconfiguring their static config (e.g. a typo when trying to reference a static cluster), although it will not break existing configs.

Testing:
None: the code path that this enables is already exercised when using the v2 config, so the existing test cases should cover this

Fixes issue #2075

cc @ggreenway

[htuch]

@ggreenway are there any implications here that are non-obvious?

@snowp I assume you really need this behavior? In general, we're pushing back on v1 enhancements but will still take them when there is a concrete need.

[ggreenway]

I don't think there's any hidden issues here; v2 already supports this behavior.

The only issue is if somebody is depending on this behavior for static config validation, but they'll have no path forward in v2 if they are anyways, so I think this is fine.