Add support for ssl_dh config

[stissot]

Description

Adds the ssl_dh attribute to config, to be able to set Diffie Hellman parameters (required to start on Ubuntu 22.04)

Generated config

# SSL DH parameters
# Generate new params with `openssl dhparam -out /etc/dovecot/dh.pem 4096`
# Or migrate from old ssl-parameters.dat file with the command dovecot
# gives on startup when ssl_dh is unset.
#ssl_dh = </etc/dovecot/dh.pem

Contribution Check List

• [x] All tests pass.
• [ ] New functionality includes testing.
• [x] New functionality has been documented in the README and metadata if applicable.

See CONTRIBUTING.md.