Add new slot prop 'providerButton' to the SignInPage component

[mkaisercross]

Summary

Should be able to pass custom props to the provider buttons in the SignInPage component. Similarly to the 'submitButton' slot prop we should have a 'providerButton' slot prop.

Examples

    `<SignInPage
      slotProps={{
        providerButton: { variant: 'contained' }
      }}
      providers={providers}
    />`

Motivation

I would like to customize the button variant to 'contained' for all the provider buttons in the SignInPage component e.g. Sign In With Github, Sign In With Facebook, etc.

Search keywords: SignInPage

[bharatkashyap]

Makes sense to me 👍🏻 We can do it as part of #4574

oauthButton might be a better name

[mkaisercross]

Great! I think oauthButton sounds better, yes.