ref(relay): More robust Relay credentials setting (#470)

This patch does two things:

1. Separate creating of Relay credentials from syncing them to Sentry config
2. Has a more flexible Relay credentials check and sync: look for the quoted public key in the config file, if it is there, assume this must be correctly set as it is very unlikely to have that random key in a different context with quotes around. The second one is to allow having other whitelisted relay keys by using an append method when adding the new key.