Load a public key PublicKey from the given stream Stream.
Supports loading both DER- and PEM-encoded keys. The key must be an RSA
or EC key. DH and DSA keys are not supported, and
PublicKey will be bound to an atom (dh_key or dsa_key) if you
try and load such a key. Otherwise PublicKey will be unified
with public_key(KeyTerm) where KeyTerm is an rsa/8
term representing an RSA key, or ec/3 for
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