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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