Move Safe Browsing key to proxy

[jumde]

Do not include the key in code or client, instead move it to the proxy. We will use anidentifier instead of the real key in the code, modify the request at the proxy to replace the identifier with the key and add rate limiting at the proxy to avoid abuse

[fmarier]

I'm not sure when this was done, but the key is in fact on the proxy. The client only has a dummy key:
https://github.com/brave/brave-core/blob/44bbdfdd8e21a1aa79f1bd473c27c46278f664d2/build/commands/lib/config.js#L95