Add a working auth with builder credentials

[augustin-v]

I have been able to interact with Polymarket's api, deriving the credentials from the private key i was given through the magic link;
but not through the builder credentials.

To implement it i think we should:

• add parameterisation of builder credentials in env
• add more customizable auth functions in case of auth using builder credentials (take inspiration from the client.rs with_l1_headers, with_l2_headers, create_or_derive_api_key functions)

test it and add a working example in the examples folder.