feat(console): let a deployment name extra sandbox entitlements hosts

[0xAlcibiades]

Closes #1558.

Part of #1486 — the auth half, direction 2 of the three that issue's thread lists.

The bind

upstream_deny_cidrs blocks the cluster service range by design, so sandboxes
cannot reach arbitrary in-cluster services. That also blocks the Console, and it
cannot be relaxed without opening everything else.

Fronting the Console on an address outside the range gets past the network
block, and then 403s. sandbox_entitlements_hosts is:

def self.sandbox_entitlements_hosts
  [ Principal.host_from_url(ENV["CENTAUR_CONSOLE_URL"]) ]
end

One host, from the Console's own env, with no config, admin API or DB row
overriding it. So the injection rule only ever matches the in-cluster address —
the one the sandbox cannot use. Reaching the Console needs a host outside the
range; getting a credential needs the host inside it. As shipped those are
mutually exclusive, which is why this is not solvable in a deployment's own
values.

Change

CENTAUR_CONSOLE_ENTITLEMENTS_HOSTS (chart:
console.entitlementsHosts) names additional hosts for the same credential.

• Adds rather than replaces. The in-cluster address keeps working, including
  for the sync path that already uses it.
• Accepts a URL or a bare host, because the value is in practice copied from
  whatever CENTAUR_CONSOLE_URL is set to, and a URL silently producing a
  non-matching rule is the failure this is meant to remove.
• Normalized through the existing Principal.normalize_hosts, so matching
  does not hinge on case, whitespace or a trailing dot, and a host that
  duplicates the default collapses.

Unset, nothing changes.

Why this direction

The issue lists three shapes. Exempting the control plane from the forwarded
deny is the narrowest, but the deny is enforced by the proxy binary rather than
by this repo, so it is not a change that can be made here. Exposing
console.extraEnv is the broadest and least targeted. This one is the smallest
change inside centaur that actually unblocks the pair.

It is the auth half only. A deployment still has to front the Console on a
reachable address; this makes that address work instead of 403ing.

Testing

Four new model tests: the default is the console URL alone; a configured host
adds to it rather than replacing it; a URL and a bare host are both accepted;
and values are normalized and de-duplicated.

I could not run them. The console suite needs bundle install and a
prepared database, and this machine has Ruby 2.6 against a Gemfile.lock wanting
bundler 4.0.10, so neither bin/rails test nor bin/rubocop would start. Both
files pass ruby -c, and filter_map/index_with are already used elsewhere
in this app, but the tests themselves rest on CI. Flagging that rather than
implying otherwise.

helm lint passes, the values schema stays valid, and helm template renders
CENTAUR_CONSOLE_ENTITLEMENTS_HOSTS only when the value is set.