adds support for text plain endpoints

[baywet]

Summary

fixes #878

Adds support for serializing/deserializing scalar values from text/plain content. A good example usage of that is /users/$count which returns the result as text plain.

int? countResult = await client.Users.Count.GetAsync(h => {
    h.Add("ConsistencyLevel", "eventual"); // this header is only required for the identity APIs
});
Console.WriteLine($"users in tenant {countResult}");

Generation diff

microsoft/kiota-samples#565

[sonarcloud]

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0 Bugs
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97.1% Coverage
0.0% Duplication

[andrueastman]

👍🏼