config: add RPC user+password, rework cookie auth

[torkelrogstad]

1. Adds support for username + password when creating the Core RPC client
2. Removes the data dir parameter, and instead adds a parameter that takes in the cookie path directly. This removes the hardcoded 'regtest' network, as a byproduct.

[octobocto]

Have tested this, works great!

[Ash-L2L]

LGTM, approved