fix(smus): dispose authprovider and then dz clients when signing out

[kzr-at-amazon]

Problem

• Signing out did not clear the DZ client, so when user re-tries signing in with corrected region, they cannot access the targeted domain.

Solution

• Dispose smuAuthProvider when signing out, within authProvider dispose, the DZ clients will also be disposed

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[vpbhargav]

Should we move this up before the refresh? In case the refresh is expected to rely on some state in AuthProvider, that will also get cleared. Line 396 maybe.