Add option for different request path (different from hardcoded '/proxy').

[optout21]

The proxy accepts RPC commands on the URL https://localhost:port/proxy, where port is configurable, but the /proxy request path is not.
Introduce a config option request_path to make this configurable:

• /proxy (or proxy) for default https://localhost:port/proxy
• / for https://localhost:port

[Joohansson]

Looks good, thanks!