Use the configured identity provider name instead of "OpenID"

### Problem

Defguard currently labels external authentication options as "OpenID".

Most end users do not know this technical term. They expect to see the name of the identity provider configured by their organisation, such as "Microsoft 365", "Okta", or "Google Workspace".

### Requested change

Where the user interface currently displays "OpenID", show the configured external identity provider name instead.

For example:

```text
Continue with Microsoft 365
```

instead of:

```text
Continue with OpenID
```

Start by searching the user interface for the existing "OpenID" label and trace how the configured external identity provider name is exposed. The work is done when the interface displays that configured name, such as "Microsoft 365", instead of "OpenID" for external authentication options.

Written by the indexing model from the issue text.