websocket target support

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According to #1081 this change adds websocket target support to monitor availability and functionality of websocket servers.

This probe provides features as follows:

• Basic HTTP authentification
• Bearer token
• Arbitrary HTTP headers
• Option to skip server certificate checking on connect
• Send/Receive sequences with regexp matching to ensure the server is working correctly ( similar to the TCP probe)

New metrics:

• probe_http_status_code: resulting http status code on connection
• probe_is_upgraded: is the connection successfuly upgraded to a websocket connection
• probe_failed_due_to_regex: 1 when send/receive sequence wasn't matched to server messages