Azure AD B2C Provider

[Azure AD B2C supports OpenID Connect](https://docs.microsoft.com/en-us/azure/active-directory-b2c/active-directory-b2c-reference-oidc) and would be a good enterprise quality service to add to Authlib loginpass.

I don't understand this library enough to add this, but I can help with some of the base information and I'm vry happy to help with live testing.

Here's a example from a **live tenant configuration URL**
```python
f'https://{tenant}.b2clogin.com/{tenant}.onmicrosoft.com/v2.0/.well-known/openid-configuration?p={policy}'
```
**Response**
```python
{
f"issuer": "https://{tenant}.b2clogin.com/{SOME_GUIDE}/v2.0/",
f"authorization_endpoint": "https://{tenant}.b2clogin.com/{tenant}.onmicrosoft.com/oauth2/v2.0/authorize?p={policy}",
f"token_endpoint": "https://{tenant}.b2clogin.com/{tenant}.onmicrosoft.com/oauth2/v2.0/token?p={policy}",
f"end_session_endpoint": "https://{tenant}.b2clogin.com/{tenant}.onmicrosoft.com/oauth2/v2.0/logout?p={policy}",
f"jwks_uri": "https://{tenant}.b2clogin.com/{tenant}.onmicrosoft.com/discovery/v2.0/keys?p={policy}",
"response_modes_supported": [
"query",
"fragment",
"form_post"
],
"response_types_supported": [
"code",
"code id_token",
"code token",
"code id_token token",
"id_token",
"id_token token",
"token",
"token id_token"
],
"scopes_supported": [
"openid"
],
"subject_types_supported": [
"pairwise"
],
"id_token_signing_alg_values_supported": [
"RS256"
],
"token_endpoint_auth_methods_supported": [
"client_secret_post",
"client_secret_basic"
],
"claims_supported": [
"emails",
"newUser",
"oid",
"sub",
"idp",
"extension_customUserAttribute",
"tfp",
"iss",
"iat",
"exp",
"aud",
"acr",
"nonce",
"auth_time"
]
}
```

Some notes:

The host URL uses tenant more than once:

```python
host = 'https://{}.b2clogin.com/{}.onmicrosoft.com/'.format(tenant, tenant)
authorize_url = '{}oauth2/v2.0/authorize'.format(host)
token_url = '{}oauth2/v2.0/token'.format(host)
jwk_set__url = '{}discovery/v2.0/keys'.format(host)
iss _url = https://{}.b2clogin.com/__SOME_GUID__/v2.0/'.format(tenant) <-- Note the GUID, not sure where that value originates from
```

Sign In, Sign Up, etc require more kwargs:
```python
client_kwargs': {
'client_id': client_id,
'nonce': '??',
'p': policy,
'response_type': 'id_token',
'scope': 'openid offline_access',
},
```
I show nonce as required, but its unclear to me if the blueprints are handling that.

If I can help in any other way, please ask.