feat: support monitoring WebSocket endpoints (#511)

* feat: support monitoring WebSocket endpoints

WebSocket endpoints are automatically identified by the URL protocol
specification: `wss://` or `ws://`. The request body is used as the
"message" written to the server, and the answer is stored in the
`[BODY]`.

Optionally, the user can set the `jsonrpc` flag to automatically wrap
the request body in a JSON RPC 2.0 method call.

* core:websocket: close connection after using it

* test: add tests related to WebSocket support

- test we can identify the endpoint type for WebSockets based on the URL
  supplied: `wss://` (with SSL/TLS) and `ws://` (plain text).
- test we can generate a JsonRPC 2.0 message via the new endpoint flag
  `JsonRPC`.

* core:endpoint: fix name of jsonrpc parameter

See https://en.wikipedia.org/wiki/JSON-RPC#Version_2.0

* core:websocket: fix dangling open connection on error

Move the `defer ws.Close()` to after opening the connection, so the
socket is closed also in case of errors.

* remove jsonrpc flag

* core:websocket: fix nil pointer dereference

The connection should only be closed if successfully opened.

* Move websocket function to client

* update go.mod

* Fix build errors

* Fix errors

* Update client/client.go

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Co-authored-by: TwiN <[email protected]>