The UpdateSequenceGenerator class introduced in SERVER-29939 randomly selects between null, true, false, and double values between -INT_MAX and INT_MAX. We should update the RandomizedScalarGenerator to be able to generate all possible BSON types in order to be able to detect idempotency issues related to different BSON types (e.g. type fidelity in $set, see SERVER-16801).

Note: When generating Decimal128 values, we should uniformly generate an exponent and generate a mantissa separately in order to be able to generate both small and large Decimal128 values with non-negligible probabilities, per geert.bosch's recommendation.