Allow specific DNSSEC key algorithms other than RSASHA256

[pecharmin]

Allows one to define another DNSSEC key algorithm then RSASHA256; based on #157.

References:

• 157 - Support DNSSEC single type signing scheme without ZSK

• https://www.iana.org/assignments/dns-sec-alg-numbers/dns-sec-alg-numbers.xhtml