specs, add test for encode integer as string

[weidongxu-microsoft]

Fix https://github.com/Azure/cadl-ranch/issues/634

Not include case of "query" and "header", because they are always string on wire.

"body property" tests on

1. safeint (int64) as required property
2. uint32 as optional property

Cadl Ranch Contribution Checklist:

• [x] I have written a scenario spec
• [x] I have meaningful @scenario names. Someone can look at the list of scenarios and understand what I'm covering.
• [x] I have written a mock API
• [x] I have used @scenarioDocs for extra scenario description and to tell people how to pass my mock api check.

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Latest commit: d9d7949906862bb2e98dd72bea68f76bd387a467

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[weidongxu-microsoft]

shall we change the folder to numeric? the previous tests for encode is named by original type name.

Make sense. Moved to encode/numeric folder (and namespace).