converts a ldns_key to a public key rr
If the key data exists at an external point, the corresponding
rdata field must still be added with ldns_rr_rdf_push() to the
result rr of this function
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k: the ldns_key to convert
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Returns ldns_rr representation of the key
AUTHOR
The ldns team at NLnet Labs. Which consists out of
Jelte Jansen and Miek Gieben.