fix issue 209: add output parameters support

[riccardos77]

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this PR address the lack of support for output parameters adding:

• sqlmock.NamedOutputArg and sqlmock.NamedInputOutputArg functions that support mapping values for sql.Named output and input/output parameters
• sqlmock.TypedOutputArg function that support mapping values for unnamed output parameters, matching by type. This is required to mock for example go-mssqldb's ReturnStatus (https://github.com/microsoft/go-mssqldb/blob/main/mssql.go). To work, the output type doesn't need to be changed, so to avoid breaking changes, I've added a PassthroughValueConverter that allows the specified types to bypass the default ValueConverter. If there is a better solution, I will be happy to improve it.

Examples have been added in examples/output folder.

This PR should completely fix issue #209.