Remove unused base64 strings in atclient_atkeys struct

[JeremyTubongbanua]

Right now, we store 4 base64 copies of the key for writing purposes, which is wasted memory if the application code never writes the atkeys.

Things to do:

• Remove base64 strings in atclient_atkeys
• Ensure that atclient_atkeys writing functionality still works. To do this, prepare mbedtls RSA context and use mbedtls_pk_write_key_pem to write it to a buffer, then write that buffer to file.

[JeremyTubongbanua]

There's some engineering needed to be done using the mbedtls_pk_write_key_pem function, so I'm assignin 3 SP.

Making the prio a P2 because these base64 strings take up a lot of heap memory.