[alertmanager-snmp-notifier] Add Gateway API HTTPRoute support

[somaz94]

This adds opt-in Gateway API HTTPRoute support to the alertmanager-snmp-notifier
chart, matching the pattern already merged for sibling charts
(prometheus-snmp-exporter #7024, jiralert #7031, prometheus-blackbox-exporter #6961).

A new templates/route.yaml renders an HTTPRoute when route..enabled is set.
It is disabled by default, so existing installs are unaffected. The backendRef
targets the chart Service on .Values.service.port (9464). This also adds a
namespaceOverride helper (consistent with the sibling charts) and a
ci/httproute-values.yaml test case.

Validation (local):

• helm template: disabled by default renders no HTTPRoute; enabled renders correctly
  across parentRefs / hostnames / filters / additionalRules / httpsRedirect variants
• ct lint: passes across all 5 ci values files (default, httproute, ingress, secret, templates)

Chart version bumped 2.1.0 -> 2.2.0.

[somaz94]

Checking in on this one — and I think I found the gate rather than just guessing at it.

@maxwo approved on 07-06 as the CODEOWNER for charts/alertmanager-snmp-notifier/, and all checks pass, but it's still showing blocked. Looking at recently merged contributor PRs here (#7143, #7118, #7116, #7107), each carries an approval from a helm-charts-admin — jkroepke or monotek — so I suspect this needs an admin approval on top of the chart-level one.

Could an admin take a look when convenient? The branch is also ~79 commits behind main now; I've deliberately not force-pushed a rebase since that just re-triggers the full CI, but say the word and I'll do it.