Add capability to pass custom parameters to the `/oauth2/token` request from the SDK level

[brionmario]

Is your suggestion related to an experience ? Please describe.

Currently, the SDK only supports adding custom parameters to the /oauth2/authorize request.

However, there may be cases where we need to send custom parameters to the /oauth2/token request as well.
ex: Attaching the tokenBindingId to facilitate client-request binding type in WSO2 Identity Server

• https://is.docs.wso2.com/en/latest/references/token-binding/client-request/

Describe the improvement

Implement a generic method to pass custom parameters to the /oauth2/token request.

Additional context

We can augment the signIn method of the wrapper SDKs (auth-spa) to accept an additional argument to get the token request config which will contain additional params.