add configuration models for additional context

[elrayle]

These are the classes to process the context configuration. They are not being used yet.

Remaining work:

• In search_config, read in and process the context configuration using these classes
• Add context_mapping_service, which will use these configurations to extract the context from the graph and return the results as json.

This PR also defines generalized helper methods for fetching configs from the config hash. This includes fetching required configs, boolean configs, and configs values that need to be converted to a symbol.

[elrayle]

• fixed typo rparam to @param
• fixed typo @returns to @return
• consistently use helper class for all access of the config hash (in new and recently refactored classes only)