endpoints: add Vault Vision endpoints

[cstockton]

This adds a function for the multi-tenant authentication platform provided by Vault Vision. It is similar to the AWSCognito function and includes a basic unit test.

[gopherbot]

This PR (HEAD: c6f73d9) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/oauth2/+/454496 to see it.

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[gopherbot]

Message from Matt Hickford:

Patch Set 1: Code-Review+1

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[gopherbot]

Message from Matt Hickford:

Patch Set 1: Run-TryBot+1

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Gopher Robot:

Patch Set 1: TryBot-Result+1

(1 comment)

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