Custom read adapter + feature flag + integration tests

[aldbr]

Description

Build a non-standard adapter (outside the regular legacy adapter pattern) that redirects the singletons' read operations to the diracx RSS API, and gate it behind a feature flag so deployments can switch between the legacy RPC backend and diracx without a code change.

Writes continue to go through the legacy RPC service by default in this phase.

Scope

• A read-only adapter used by ResourceStatus and SiteStatus to fetch status from diracx instead of the RSS RPC service.
• A feature flag (CS toggle) to switch between the DIRAC and diracx backends at runtime.
• Certification tests that exercise the singletons end-to-end against a diracx backend, covering the main consumer patterns: element status for SEs / CEs / FTS, and the site queries (getSiteStatuses, getUsableSites, getSites).

Non-goals

• No changes to the write path: writes still go to the legacy RPC service.
• No removal of the legacy RSS RPC service.

Acceptance criteria

• Feature flag switches the singletons between legacy RPC and diracx backends.
• With the flag on, reads are served by diracx and writes continue to reach the legacy RPC service.
• Behavior is verified against a certification setup.