Configuration management refactoring

[marcofavorito]

Is your feature request related to a problem? Please describe.
The implementation of configuration management could be simplified and cleaned up.

Among other things:

• Simplify configuration loader instantiation.
• Implement proper custom loader, instead of wrapping it at import time.
• Implement PyPI dependency class (borrow from pip?), instead of raw dict type.
• add protocol specification as separate configuration class (and not inheriting from ProtocolConfig), change and clean up the loading of a protocol specification.
• the key-order-preserving YAML loading is a bit hacky; that could be made neater as well
• fix the discrepancies introuced in [AEA-506] Handle multipage AEA configuration  #1674 between AgentConfig and the other configuration classes. In particular, the first expects a multi-paged YAML file, which contrasts with the from_json/to_json interface, which only expects a JSON object (and not a list)
• in tests, make validators as fixtures.

Describe the solution you'd like
Address some of the above.

Describe alternatives you've considered
n/a

Additional context
low priority.

[DavidMinarsch]

Related #969

[solarw]

some thoughts:

1. Change internal configuration representation. use dict/mapping instead of variables inside the Configuration classes
   to make it more from/to json compatible, all the properties will stay the same, init method the same,
   the primary goal is to unify: to/from json methods on base class, instead of writing it for every class.
   also, dict/mapping inside the config can help to use some yaml friendly mapping to preserve order while dealing with configs

2. Use AgentConfigManager for loading/accessing/modifying agent-wide configurations including overrides.
   just to have a single point of responsibility with all validation in one place. so every time config for aea project loaded from the file system, it's better to handle it with config manager, to avoid direct modifications.

3. would be nice to find the way to unify agent configuration properties with builders some, to avoid code duplication when new aea configuraion option is added.