Allow users to escape newlines in p8 certs

[peterferguson]

When trying to deploy on fly I was facing an issue with the reading of the apns .p8 cert.

I couldn't find a good way to securely store the cert so it could be read in the application so that just leaves an env var.

However when you do the naive thing setting APNS_P8_CERTIFICATE to the .p8 file contents the server will fail with an error initing client.Production()

This pr allows the user to set the env by explicitly escaping the newlines:

If the .p8 cert is

"-----BEGIN PRIVATE KEY-----
flkdflkdf...
-----END PRIVATE KEY-----"

then you can set the env var as follows

APNS_P8_CERTIFICATE="-----BEGIN PRIVATE KEY-----\nflkdflkdf...\n-----END PRIVATE KEY-----"

If I am just missing a better way to do this please let me know 🙏

[nplasterer]

This seems like a fine fix to me. Just seeing if @zombieobject who is more familiar with iOS has any thoughts.