L402: streaming response gRPC calls don't support creating L402 token

Nobody has claimed this yet.

For some reason, when the server sends back the `402 payment required` response code if the user doesn't already have an L402 token, the [client interceptor code](https://github.com/lightninglabs/aperture/blob/709463fe5b01b0aa4c0c60b3a8eabdfbe6db4ee6/l402/client_interceptor.go#L172) doesn't detect that properly and doesn't attempt to create a token, resulting in a terminal error.

It is yet unclear where exactly the problem lies, the following possibilities come to mind:
- The server's response status fields aren't set correctly for the client interceptor to recognize: https://github.com/lightninglabs/aperture/blob/b1b0dc557071fffd1580772a006037e8a4f6203b/proxy/proxy.go#L466. Perhaps the gRPC/HTTP/2 specific trailer fields need to be used?
- The client library just doesn't look for the response code [in the right place](https://github.com/lightninglabs/aperture/blob/709463fe5b01b0aa4c0c60b3a8eabdfbe6db4ee6/l402/client_interceptor.go#L195) (it interprets the error from the stream interceptor, but perhaps it needs to look at response header/trailer fields?)
- This is a limitation of the gRPC Golang library itself and cannot be fixed easily.

In any case, it should be pretty straightforward to set up a new unit test (taking https://github.com/lightninglabs/aperture/blob/b1b0dc557071fffd1580772a006037e8a4f6203b/proxy/proxy_test.go#L322 as a template) that executes the streaming case and allows for local debugging of the issue.

## Workarounds

The current workaround in most Lightning Labs application is to make sure we call a non-streaming (unary) gRPC method first, which then triggers the L402 token creation and allows subsequent streaming gRPC methods to work.

Start with the client interceptor in l402/client_interceptor.go and the response handling in proxy/proxy.go, then use proxy/proxy_test.go around the referenced test as a template for a streaming case. Debug where the 402 status is exposed to the interceptor; done means a streaming call without an L402 token detects the response and creates one instead of returning a terminal error.

Written by the indexing model from the issue text.