tls: Add distributed_stek module

This migrates a feature that was previously reserved for enterprise
users, according to https://github.com/caddyserver/caddy/issues/2786.

TLS session ticket keys are sensitive, so they should be rotated on a
regular basis. Only Caddy does this by default. However, a cluster of
servers that rotate keys without synchronization will lose the benefits
of having sessions in the first place if the client is routed to a
different backend. This module coordinates STEK rotation in a fleet so
the same keys are used, and rotated, across the whole cluster. No other
server does this, but Twitter wrote about how they hacked together a
solution a few years ago:
https://blog.twitter.com/engineering/en_us/a/2013/forward-secrecy-at-twitter.html

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