Add ability to refresh the user identity in useGetIdentity

[fzaninotto]

Problem

If an application contains a form letting the current user update their name and/or avatar, developers may want to refresh the identity after the form is submitted. It is currently cumbersome as it implies adding a context on top of the app (cf https://marmelab.com/blog/2020/12/14/react-admin-v3-userprofile.html)

Solution

Use react-query to fetch the authProvider in useGetIdentity

Sample code

const IdentityForm = () => {
    const { isLoading, error, identity, refetch } = useGetIdentity();
    const [newIdentity, setNewIdentity] = useState('');

    if (isLoading) return <>Loading</>;
    if (error) return <>Error</>;

    const handleChange = event => {
        setNewIdentity(event.target.value);
    };
    const handleSubmit = (e) => {
        e.preventDefault();
        if (!newIdentity) return;
        fetch('/update_identity', {
            method: 'POST',
            headers: { 'Content-Type': 'application/json' },
            body: JSON.stringify({ identity: newIdentity })
        }).then(() => { 
            // call authProvider.getIdentity() again and notify the listeners of the result,
            // including the UserMenu in the AppBar
            refetch();
         });
    };
    
    return (
        <form onSubmit={handleSubmit}>
            <input defaultValue={identity.fullName} onChange={handleChange} />
            <input type="submit" value="Save" />
        </form>
    );
};

[fzaninotto]

I considered exposing the refetch from react-query in the useGetPermissions response instead. I still hesitate, and I'm open to comments.

Edit: I finally opted in to using refetch, it's simpler to use and explain.

[slax57]

Minor changes requests, otherwise I approve the main changes

[smeng9]

Removing typeof authProvider.getIdentity === 'function' check causes a tiny BC break.

We found there are console errors after upgrading pass 4.6 because we have never implemented the getIdentity method since it was introduced in #5180

In the previous pull request it says

The change is backward compatible (if the authProvider.getIdentity() method isn't implemented, the admin shows an anonymous user avatar with no name, as before).

In v4 upgrade guide it did not mention this method is now required to be implemented. I believe we should now add a note about that breaking change.

[smeng9]

It will be very easy for us to catch the issue and implement the method.

Just for other users they should be aware when they upgrade.

[slax57]

Thanks for pointing that out!
The getIdentity function should remain optional, even in v4, so I believe we should reintroduce this check to avoid errors. I'll open a new PR.

[slax57]

Here it is: #8509