Add ability to link user account with existing account

[jordond]

This PR allows Firebase accounts to be linked. The feature is toggleable because Apple requires getting explicit permission when linking Apple accounts to other accounts.

[mirzemehdi]

Thank you for this amazing PR @jordond. All looks good. But looks like feature is incompleted. Currently, it supports only linking apple sign-in, but for github and google sign in linking is not supported. Can you please also add support for that?

[mirzemehdi]

Thank you @jordond for initiating this amazing PR.

Now, all of sign in ui containers have optional linkAccount field. By default it is false as before

//Google Sign-In Button and authentication with Firebase
GoogleButtonUiContainerFirebase(onResult = onFirebaseResult, linkAccount = false) {
    GoogleSignInButton(modifier = Modifier.fillMaxWidth().height(44.dp), fontSize = 19.sp) { this.onClick() }
}

//Apple Sign-In Button and authentication with Firebase
AppleButtonUiContainer(onResult = onFirebaseResult, linkAccount = false) {
    AppleSignInButton(modifier = Modifier.fillMaxWidth().height(44.dp)) { this.onClick() }
}
        
//Github Sign-In and authentication with Firebase
GithubButtonUiContainer(onResult = onFirebaseResult, linkAccount = false) {
    Button(onClick = { this.onClick() }) { Text("Sign In with Github") }
}