Add environment variable support for `server_metadata_url`

**Is your feature request related to a problem? Please describe.**

Currently, authlib supports the following environment variables for OAuth configurations:

```text
{name}_CLIENT_ID
{name}_CLIENT_SECRET
{name}_REQUEST_TOKEN_URL
{name}_REQUEST_TOKEN_PARAMS
{name}_ACCESS_TOKEN_URL
{name}_ACCESS_TOKEN_PARAMS
{name}_AUTHORIZE_URL
{name}_AUTHORIZE_PARAMS
{name}_API_BASE_URL
{name}_CLIENT_KWARGS
```

However, when working with OIDC providers that support discovery endpoints (like `.well-known/openid-configuration`), we need to manually configure the `server_metadata_url` in code rather than through environment variables.

**Describe the solution you'd like**
It would be great if authlib could also support:

```text
{name}_SERVER_METADATA_URL: Server metadata discovery endpoint URL
```

Reference
https://docs.authlib.org/en/stable/client/flask.html#configuration