To save up some space and only bring the library users the necessary clients it would be possible to hide some of them behind Rust features. Also by doing this the users could get rid of at least some of the dependencies of the library. See https://doc.rust-lang.org/cargo/reference/features.html as a reference. The features could be split per API products (accounts, payment, corporate-payment etc.)

Add API and model for Holdings https://op-developer.fi/docs/api/3VZiIRoT2EowKC6yeA0gqQ/Holdings

Also add Client interfaces and tests.

To prevent actual calls to OP API, introduce a mock for the HTTP requests. See https://docs.rs/mockito/0.27.0/mockito/ as an example. The payloads for the responses should match what is returned from the actual API for different endpoints (use for example Postman to get these and save them as json to test resouces to be used with mock).

Create examples directory for this so it can be run with cargo run --example command. The example should take the API key as command line parameter and then fetch data from the OP API sandbox and print it out to console.

Add new API to get branches information with query parameters. See https://op-developer.fi/docs/api/1cVuWWotfMaks6oQQ0YOea/Branches

Also add interfaces for Client and tests.

Add code examples to accounts rust documentation using markdown formatting. The example should show how each function of the client is supposed to be used

Add new API and models for https://op-developer.fi/docs/api/5lWcjqy3JY2G2y4UGmS6Yw/Funds-data
Add also tests + client interface changes.