Allow options customization

[probert94]

A while ago Google introduced the immediate mediation (Explainer) which allows to open a credentials chooser (browser ui) only if "immediate" credentials (stored password or directly available passkeys) are available.
This way the typical loginforms, showing user + password as well as buttons for passkey and social login could be skipped.
In a recent blog post Google announced that this feature is available in Chrome 149 but the immediate mediation has been replaced by an additional option uiMode which has to be set to immediate.
Unfortunately you are not yet able to use this feature, if you are using @SimpleWebAuthn/browser.
As this API is still experimental and supported only by Chromium I don't expect it to be added to SimpleWebAuthn directly.
What I would like to suggest instead is to allow customization of the options passed to navigator.credentials.get and navigator.credentials.create by passing a callback function. This way, new APIs or APIs out of scope of this library (like FedCM) could be used while still benefiting from the simplification of this library.

Related discussion:
#740