tls.go/arguments: Accept --signature-algorithms list

[justinbastress]

We have a placeholder for this, but because of (https://github.com/zmap/zgrab2/blob/master/tls.go#L164), this is not implemented.

Requires a corresponding update to zcrypto to (a) expose the necessary types / functions / constants, and (b) make use of them -- see https://github.com/zmap/zcrypto/issues/116.

Take a comma-separated list of hex values, maybe ((signature << 8) | hash)?