feat: Bypass server-side filtering for server keys

[MCPx]

As suggested by @matthewelwell in this issue, this workaround allows for adding server keys to the edge proxy that will bypass filtering of server-side only features, e.g.

"environment_key_pairs": [
    {
      "client_side_key": "abc123",
      "server_side_key": "ser.def456"
    },
    {
      "client_side_key": "ser.def456",
      "server_side_key": "ser.def456"
    }
  ],

Tests added to ensure that the three places that use filter_out_server_key_only_feature_states are covered and confirmed to be returning the feature that is filtered out for a client side key.

[khvn26]

LGTM 👍