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Reduce copy&paste.

docs: TLS server config clarification

See merge request knot/knot-resolver!718

Experimental DNS-over-TLS to auth module

See merge request knot/knot-resolver!711

This modules allows knot-resolver to discover authoritative servers SPKI
digest by leveraging magic NS target names ala DNSCurve.

This commit allows opportunistic DNS over TLS to origins configured
as supporting DoT on port 853. It also adds interface for clearing
configured TLS clients to allow runtime reconfiguration.

The general mode of operation is as follows:

1. Produce a new outgoing query
2. Check if the selected upstream address has configured TLS support on port 853
 2a. If it does: upgrade to DNS over TLS, it cannot be downgraded from this point
 2b. If not: continue with preferred protocol

This allows further automatic discovery as in [1], but right now it has to be configured
manually.

[1]: https://tools.ietf.org/id/draft-bortzmeyer-dprive-resolver-to-auth-00.html

(cherrypicked from cloudflare branch, need to be adapted)

transport tests

See merge request knot/knot-resolver!707