Add support for separate endpointUrlMain and endpointUrlSandbox metadata keys for deployment endpoint overrides

Description

Currently, the platform API accepts three metadata keys when creating/overriding a deployment:

• endpointUrl - Overrides the upstream endpoint URL
• vhostMain - Overrides the virtual host for the main environment
• vhostSandbox - Overrides the virtual host for the sandbox environment

Since an API can have both a main and a sandbox upstream endpoint, it should be possible to override them independently at deployment time, just as vhosts can be overridden independently.

Proposed change

Introduce two new metadata keys alongside the existing one

• endpointUrlMain — overrides Upstream.Main.URL
• endpointUrlSandbox — overrides Upstream.Sandbox.URL

The existing endpointUrl key should either be kept as an alias for endpointUrlMain (for backwards compatibility) or deprecated.

Affected areas:

• constants/constants.go — add MetadataKeyEndpointUrlMain, MetadataKeyEndpointUrlSandbox
• service/deployment.go — read and apply both new keys; extend applyEndpointOverride (or split it) to handle sandbox URL
• dto/upstream.go / gateway YAML structs — verify sandbox upstream URL field is writable through the same path
• No Gateway side changes needed

Start in constants/constants.go and service/deployment.go, then trace dto/upstream.go and the gateway YAML structs to verify how upstream URLs are written. Done means endpointUrlMain and endpointUrlSandbox independently override the main and sandbox URLs, while endpointUrl remains an alias or is deprecated; no Gateway changes are needed.