Add query that lists SAML external identities for an Organization

Business Plan hosted customers with SAML would like a way to programmatically query information about a GitHub user account's SAML identity. The following is a GraphQL query that accomplishes this for an Organization.

- [ ] Determine whether this is a standalone example for useful Business Plan hosted (SAML) queries or added under https://github.com/github/platform-samples/tree/master/graphql/queries

```
query {
organization(login: "LOGIN") {
samlIdentityProvider {
ssoUrl
externalIdentities(first: 100) {
edges {
node {
guid
samlIdentity {
nameId
}
user {
login
}
}
}
}
}
}
}
```

/cc @francisfuzz as you might be able to track down the author to see if they want to add this ☝️

Start by reviewing the graphql/queries directory and the issue discussion to decide whether this belongs there or as a standalone example. Use the provided organization SAML externalIdentities query as the reference, and consider the work complete when the example is placed in the agreed location and can be used to list the external identity GUID, nameId, login, and SSO URL.

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