Creating new filtered SAML linked identity queries for GHEC non-EMUs

[stacycarter]

This PR creates two new sample GraphQL queries for GHEC non-EMUs:

• org-saml-identities-filtered-by-nameid-username.graphql
• enterprise-saml-identities-filtered-by-nameid.graphql

These queries can be used to find a stored external identity that has a specific SAML nameId or SCIM userName value.

For example, this can help an enterprise or organization owner identity the root cause when a user receives the error message below (basically, it can show which GitHub user is already linked to the external identity in the organization/enterprise):

Your GitHub user account [GitHub username] is currently unlinked. However, you are attempting to authenticate with your Identity Provider using [IdP user account] SAML identity which is already linked to a different GitHub user account in the [organization/enterprise]. Please reach out to one of your GitHub [organization/enterprise] owners for assistance.

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[johnaAr555]

You will need to replace <ENTERPRISE_SLUG> and with the actual GitHub enterprise slug and the SAML NameID value that you're searching stored external identities for in the GitHub enterprise.

For GitHub Enterprise Cloud enterprises that have SAML configured at the enterprise level, this will query the stored SAML nameId external identity values in the GitHub enterprise, and if one is found that matches the value specified for <SAML Name ID>, it will print out the SAML nameId and GitHub username for that stored external identity.