Make `DocumentAuthority` related APIs public from `SchemaSystem`

[desaikd]

Problem statement

Below are few use cases for which this change is required:

• In a situation, where user wants to load all the schemas for a document authority in their schema system. Instead of iterating through the authorities that user initially used to set the SchemaSystem, they can just do SchemaSystem#authorities(), to get those authorities.
• User may want to modify the authority path for when they change their location of storing the schema files. (e.g.with_authorities, add_authority)

Currently the above mentioned APIs are present under SchemaSystem but are private APIs, modify these to be public APIs instead.