Support importing SAML metadata xml to configure SSO

### Description

To setup SAML SSO requires three pieces of configuration from the IdentityProvider; the entityID, the SSO url and the public certificate.

Whilst these can be individually copied into the configuration, there is a standard XML metadata format that many IdPs provide that includes this information.

It would be a small paper-cut improvement to support importing this metadata when setting up an SSO configuration.

Here's an example from OKTA:

```

... a certificate ...



urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress

```

### Which customers would this be available to

Enterprise Tier Only (EE)

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate