Unify TLS settings in prometheus/common

[bboreham]

We have server-side TLSConfig in exporter-toolkit/web, and client-side TLSConfig in common/config.
Exporter-toolkit is marked as "currently WIP and experimental".

We should factor it out, enable common config changes, e.g. disable TLS 1.0 more easily.

At the Prometheus 2023 Dev Summit 06 meeting it was agreed to move the TLS handling into common.

Ref #157.