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 ☝️