Inconsistent method visibility in ClientSchema

The `org.apache.ignite.internal.client.table.ClientSchema` class has some inconsistencies regarding method visibility that may hinder it's future usage:

For instance:

• ClientSchema#column is public, but ClientSchema#columnSafe is package-private.
• ClientSchema#getMarshaller(Mapper) is public, but ClientSchema#getMarshaller(Mapper, Tuple) is package private; #getMarshaller(Mapper, TuplePart, allowUnmappedFields) is also package-private but we might want to hide the logic of allowing unmapped fields.