add HTTP proxy support

[dyc3]

• allow extracting the http client from web api transport
• refactor most commands so that they can accept an external transport
• update confirmer to use transport's http client
• update remove command
• update TwoFactorClient so it doesn't need to be mutable
• update get_server_time so it requires a transport
• update remove_authenticator
• update login proceedure to use given transport
• update usages of do_login
• update setup
• update trade
• update code command
• update qr-login command
• make borrowcheck happy
• make WebApiTransport Clone and remove Default impl
• remove dead code
• fix lints

closes #177