[SYSTEMDS-3955] Add signed SSL certificates support

[gaturchenko]

This PR adds support for reading in a signed certificate and makes the coordinator verify it in place of the existing self-signed certificate generation. To that end, the XML config is extended with the following:

Key	Side	Comment
sysds.federated.ssl.cert	worker	X.509 certificate chain, PEM, leaf first
sysds.federated.ssl.key	worker	matching private key, PKCS#8 PEM
sysds.federated.ssl.keyPassword	worker	only if the key is encrypted
sysds.federated.ssl.trust	coordinator	certificates trusted to sign worker certificates, typically the CA
sysds.federated.ssl.hostnameVerification	coordinator	true by default

Both certificate and key are read by each worker from its own config, so they are local per worker. The coordinator has a single trust file, where one CA certificate covers any number of workers, and multiple CAs can be concatenated into one PEM. With hostname verification on, a worker's certificate must be issued for the address the script connects to (san=dns:..., or san=ip:... for IP literals), so a certificate valid for one worker cannot be replayed for another.

NB: private keys and certificates are added as a part of this PR for SSL-enabled tests to work. They are generated with a dedicated shell script and are irrelevant for anything except the tests.