Add 'certs' option to TLS block for multi-certs support

[wallyqs]

This adds a certs option to the tls block to support loading multiple certs:

    tls {
      certs = [
        {
          cert_file: "./configs/certs/srva-cert.pem"
          key_file:  "./configs/certs/srva-key.pem"
        },
        {
          cert_file: "./configs/certs/srvb-cert.pem"
          key_file:  "./configs/certs/srvb-key.pem"
        }
      ]
      ca_file: "./configs/certs/ca.pem"
      verify:  true
      timeout: 2
    }

Follow up from #2029

[philpennock]

What happens if given a config block with a present-but-empty list of certificates?

tls {
  certs: []
}

[philpennock]

tiny nit, %q will already quote, so the square brackets seem redundant

[wallyqs]

What happens if given a config block with a present-but-empty list of certificates?

tls {
  certs: []
}

that would have been the same as doing this:

tls {
}

which also makes the server start with TLS 'configured' but actually cannot be used. I think we should change to prevent this?
In releases so far doing tls {} would result in this:

[86149] 2023/12/15 11:27:44.329219 [INF] TLS required for client connections
[86149] 2023/12/15 11:27:44.329509 [INF] Server is ready
[86149] 2023/12/15 11:27:59.796999 [ERR] 127.0.0.1:62606 - cid:5 - TLS handshake error: tls: no certificates configured

I think this needs a separate bug fix, since need to consider the leafnode TLS remote case where no cert is configured but want to change the tls timeout:

		  remotes = [
		    {
		      url: "tls://nats:7422"
		      tls {
		        timeout: 5
		      }
		    }
		  ]

[derekcollison]

LGTM - Minor nit