Precompiled queries: pregenerate relational commands via IRelationalCommandBuilderFactory

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The current implementation of SQL pregeneration for precompiled queries instantiates RelationalCommand directly; although providers can in theory have their own implementations, no relational provider currently makes use of this. Doing this would require changes to the IRelationalCommandBuilder interface, which currently only supports incrementally building up a relational command by gradually appending SQL and parameters one by one.