Implement local DM web service for access to firewalled services

Without this service, all local services relating to data management and access (such as SODA) will need to be publicly accessible by the central DM API service. If these are required to be firewalled in some cases, then a local version of the 'DM API' (identified as a compulsory service in the implementation plan) will be required. This must be developed promptly in PI24 so that sites have time to deploy the service on their infrastructure.

AC1: Code for new service developed and committed to new repository in GitLab. This service should be able to proxy data access requests to an internal (possibly firewalled) service, subject to a permissions check.

AC2: Demo and operator documentation suitable for stakeholders and operations team tasked with deploying at the SRCNet sites.

Code (incl Helm chart): https://gitlab.com/ska-telescope/src/src-dm/ska-src-dm-da-service-gatekeeper/-/tree/main?ref_type=heads

Operator docs: https://ska-telescope.gitlab.io/src/kb/ska-src-docs-operator/services/local/mandatory/authorization/gatekeeper.html

Demo: https://confluence.skatelescope.org/pages/viewpage.action?pageId=294228024