fix issue 209: add output parameters support

[riccardos77]

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.

[riccardos77]

Hi team,

Just a friendly reminder to review this pull request when you have a moment. Your feedback is much appreciated.

I really use this library a lot and I need these changes... I would like to avoid using replace that point to a fork

Thank you!