Support PEM-PKCS#1 besides PEM-SPKI for RSA public keys

[dahlia]

Although the vast majority of ActivityPub software encodes RSA public keys in PEM-SPKI format, some software encodes RSA public keys in PEM-PKCS#1 format (see: fedify-dev/hollo#109 (comment)). Fedify currently only accepts PEM-SPKI format, so it needs to accept PEM-PKCS#1 format as well for better interoperability.

[dahlia]

Fortunately, using Node.js's node:crypto module, it is possible to convert an RSA public key encoded in PEM-PKCS#1 to PEM-SPKI.