CSHARP-5453: Add builder for CSFLE schemas

[papafe]

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[papafe]

@sanych-sun @damieng Can you give a look at the current state of the API? It's mostly functioning too, but I would like to get some feedback before writing tests.
Some notes:

• At the moment the naming is derived from the name the fields have in the json schema, as presented in the documentation(here and here).
• I taken examples mostly from the documentation pages, with some differences regarding the keyId.
• I'll take care of the xml docs only at the end.

[papafe]

@sanych-sun @damieng A couple of notes for the review:

• I have decided to reuse EncryptionAlgorithm and not create a new enum, even though EncryptionAlgorithm contains some values that are not valid for the schema, but will throw an exception.
• bsonType is actually optional, differently from what we discussed together. For this reason I slightly modified the overloads to avoid ambiguities when calling the methods.
• The whole encrypt section of a field could be empty. This is supported in the PR