SSH key usage

Hello,

First of all, **thank you** for your work, I'm using age for years now to encrypt backups and data. It works very well!

I started a new project and this library answers my need because I would have liked to be closed to Typescript ecosystem.

I understand age supports several system to encrypt data like passphrase, generated identity but it also supports ssh key (RSA). I would like to use a public RSA key with this library and I met issues.

#### code
`e.addRecipient(recipient);` // recipient is a public key, the same I use for `age -r public key [...]`
- `String must be lowercase or uppercase` (so I added `toLowerCase()`)
- `Unknown letter: "=". Allowed: qpzry9x8gf2tvdw0s3jn54khce6mua7l`

I guess either this library does not support RSA key either it's not implemented.

I can generate identity and recipient with provided helpers but I would also really like use SSH keys (RSA and ed25519). As I'm not confortable with this, I would like to know your opinion about that and what are options/solutions.

Thanks!