Add allow_insecure_auth to opt out of the HTTP+auth guard

[hashhar]

Description

Fixes #494.

The client rejects credentials over plain http://, with no opt-out. This breaks topologies where the transport is encrypted below the application layer, e.g. an mTLS-terminating service mesh sidecar: the connection looks like plain HTTP but isn't insecure end-to-end.

Add allow_insecure_auth (default False) to suppress the guard. If the coordinator has HTTPS enabled, Trino's own http-server.authentication.allow-insecure-over-http must also be set server-side.

Release notes

(x) Release notes are required, with the following suggested text:

* Add an `allow_insecure_auth` connection option to opt out of the guard
  against sending authentication credentials over plain HTTP for
  connections that are encrypted below the application layer.
  ({issue}`494`)