add proxy feature

[kimsung604]

No description provided.

[phildini]

Hi! Could we get a bit more on what this PR is trying to solve? Thanks.

[kimsung604]

Sometimes, due to security policy issues, Web Application Server cannot directly access the external network, so you must only access the external network through the Proxy Server.
So there needs to be a way to call the patreon api through a proxy server.
like this

Proxy proxy = new Proxy(Type.HTTP, new InetSocketAddress("proxy.server1.com", 19002));

// auth
PatreonOAuth patreonOAuth = new PatreonOAuth(clientId, clientSecret,redirectUri, proxy);
patreonOAuth.getToken(code);

// api
PatreonAPI api = new PatreonAPI(accessToken, proxy);
api.fetchCampaigns();
...