Add ability to set basic auth as an authentication option for requests

[cguess]

This PR adds the ability to set basic_auth credentials for requests. This is useful if, for instance, you put an Ollama instance behind a NGROK end point and want to add a bit of security to it.

To use there's two options

1. By setting the username and password separately in a hash

require 'ollama-ai'

client = Ollama.new(
  credentials: {
    address: 'http://localhost:11434',
    basic_auth: { username: 'usrname', password: 'passwd' }
  },
  options: { server_sent_events: true }
)

2. By setting a string

require 'ollama-ai'

client = Ollama.new(
  credentials: {
    address: 'http://localhost:11434',
    basic_auth: 'usrname:passwd'
  },
  options: { server_sent_events: true }
)

I've updated the README as well. There doesn't seem to be any tests for this repo, but I've tested it locally in my own codebase and it works as indicated.