add shorthand functions for serialization

[baywet]

to facilitate serialization and deserialization of models and collections of models, we're adding static helper functions.
More details to be found at microsoft/kiota#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

[Ndiritu]

Customers that could benefit from this
microsoftgraph/msgraph-sdk-php#1556
microsoftgraph/msgraph-sdk-php#1471