add shorthand functions for serialization

Inizia leggendo i requisiti in microsoft/kiota#3406 e ispeziona le astrazioni esistenti per la serializzazione/deserializzazione di modelli e collezioni. Definisci gli helper supportati per stringhe e stream, le varianti di content-type e JSON, i percorsi di reflection o callback, e conferma che l’intera API sia coperta senza conflitti di nomi.

to facilitate serialization and deserialization of models and collections of models, we're adding static helper functions.
More details to be found at https://github.com/microsoft/kiota/issues/3406

For serialization we need:

• Serialize(contentType, model): string
• Serialize(contentType, model): stream
• SerializeCollection(contentType, models): string
• SerializeCollection(contentType, models): stream
• Serialize(model): stream (maps to same one passing the json content type as a constant)
• Serialize(model): string (same)
• Serialize(models): stream (same)
• Serialize(models): string (same)

Note: if converting from stream to string is trivial we can ommit the string overloads
Note: if another parameter for the serialization method needs to be added, feel free to do so
Note: if both a string and and stream method need to be added, but can't have the same name, name the string methods with AsString suffix
Note: if the Json methods and the ones accepting the same content type can't have the same namde/would need to be aliased in the consuming code to avoid conflicts, as a ToJsonSuffix (full convention with string SerializeCollectionToJsonAsString)

For deserialization we need:

• Deserialize(contentType, stream, createTFromDiscriminatorMethod): T
• Deserialize(contentType, string, createTFromDiscriminatorMethod): T
• Deserialize(contentType, stream): T (uses reflection, only if possible, calls into the previous one)
• Deserialize(contentType, string): T (same)
• DeserializeCollection(contentType, stream, createCallback): T[]
• DeserializeCollection(contentType, string, createCallback): T[]
• reflection version x2
• Json equivalent with constant for all the previous

Note: the same notes as serialization apply
Note: if the collection and single methods can have the same name, make them same