feat(helm): add location snippet to nginx config

[joschi36]

What this PR does / why we need it:

The current Helm chart for the Loki gateway does not provide a way to inject Nginx configuration directives inside individual location blocks. This makes it impossible to implement common and important authentication or header manipulation schemes that rely on directives like proxy_set_header without overriding the entire Nginx configuration file.

A key use case is mTLS-based multi-tenancy, where an X-Scope-OrgID header must be set based on client certificate details. Due to Nginx's directive inheritance rules, setting this header in serverSnippet is overridden by any location block that defines its own proxy_set_header (e.g., for X-Query-Tags in the /loki/api/v1/ location or WebSocket upgrades in the /loki/api/v1/tail location). This results in failed authentication for some endpoints but not others.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

This change is fully backward-compatible. If locationSnippet is not set, the template renders exactly as it did before, resulting in no change for existing users.

This was tested by implementing the mTLS multi-tenancy scheme described above, which was previously only possible via complex workarounds like overriding the entire nginx.conf file or using a post-renderer. With this change, the configuration becomes trivial and is managed cleanly through values.yaml. The snippet has been added to all location blocks for consistency and to ensure all API endpoints served by the gateway are covered.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR

[CLAassistant]

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[joschi36]

Seems to be like a duplicate of #11348

Can we go forward with either one?
Customizing the clientMaxBodySize would benefit us as well