FLE: Optionally read @Encrypted POJO properties from unencrypted JSON fields.

Let the @Encrypted annotation read from unencrypted fields, so users can add encryption incrementally. (Previously, the read would fail).

Make it optional because enabling this feature would let an adversary to forge a field value simply by replacing an encrypted value with an unencrypted one.

Suggested release note text:

The `@Encrypted` annotation can now be used to migrate an existing field from unencrypted to encrypted. If you annotate a field with

@Encrypted(migration = Encrypted.Migration.FROM_UNENCRYPTED)

then either encrypted or unencrypted values will be accepted during deserialization.