Users can have multiple identities at multiple authentication providers, we should allow users to link there User with another set of credentials.

When implementing linkWithRedirect we should use a different header than "X-Stitch-Location" which was used in the previous Stitch SDKs https://github.com/mongodb/stitch-js-sdk/blob/310f0bd5af80f818cdfbc3caf1ae29ffa8e9c7cf/packages/browser/core/src/core/auth/internal/StitchAuthImpl.ts#L150-L184
(https://jira.mongodb.org/browse/RJS-606)