Fix JWK parsing when a non-standard exponent is used

[michaelpaliy]

Related to https://github.com/istio/istio/issues/41729

Currently, there is a validation to check that the exponent is equal to 65537 or 3: https://github.com/google/jwt_verify_lib/blob/master/src/jwks.cc#L107 but the exponent can be any number > 2.

issue https://github.com/google/jwt_verify_lib/issues/96

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