Expose interface for compiling arbitrary state preparation operations

[cgranade]

The function Microsoft.Quantum.Preparation._CompileApproximateArbitraryStatePreparation does not adhere to current design principles, and thus has not been exposed for public use. Currently, this function cannot be converted to use the new internal keyword, however, as the QML library uses this function in its input encoding logic. The relevant compilation logic should be exposed publicly, such that the QML input encoding logic can depend on the new functionality.