BUG: Firebase Auth/Import Users: a phone number field is missing in a user identity provider struct

### Environment
* Operating System version: macOS 13.2.1
* Firebase SDK version: Firebase Admin SDK v4
* Library version: v4.10.0
* Firebase Product: auth

### A problem
We actively use Firebase Auth in a new project.
Some tools and services are written in Go.
We want to migrate existing users to Firebase.
So we use Firebase Auth's Import Users feature.
Almost all works well, but it is hard to test some cases because some info is missing in the docs.
That is why being looking through the Firebase Auth docs for other programming languages (https://firebase.google.com/docs/reference/admin/node/firebase-admin.auth.userproviderrequest.md#properties)
we figured out that a `phoneNumber` field is missing in the `auth.UserProvider` struct in Go version of the sdk.

### Reproduce
Just take a look at Node.js' docs https://firebase.google.com/docs/reference/admin/node/firebase-admin.auth.userproviderrequest.md#properties
and at source codes of Go version https://github.com/firebase/firebase-admin-go/blob/1d245771f556db75b76b32a793b931be6f39c630/auth/import_users.go#L195

### Relevant Code
The `UserProvider` struct should have all the fields that Node.js version has:
```go
type UserProvider struct {
UID string `json:"rawId"`
ProviderID string `json:"providerId"`
Email string `json:"email,omitempty"`
PhoneNumber string `json:"phoneNumber,omitempty"` // missing field in current version
DisplayName string `json:"displayName,omitempty"`
PhotoURL string `json:"photoUrl,omitempty"`
}
```
A use case:
```go
if p.Phone != nil {
providers = append(providers, &auth.UserProvider{
UID: p.ID.String(),
ProviderID: ProviderPhone,
PhoneNumber: *p.Phone,
DisplayName: displayName,
})
}
```
A main idea is the same fields should be used across all the implementations of the Firebase Admin API.