fpm_model_t serialization

[perazz]

Description

As discussed here, it is desirable to have fpm be able to fully serialize/deserialize fpm_model_tvariables to/from TOML or JSON.
Relevant discussion at 40:25 and 42:55 mins

Possible Solution

• create an abstract serializable_t class which implements the serialization APIs
• enforce all relevant classes inside fpm_model_t to extend serializable_t
• write implementation routines for each of them

Additional Information

• If the fpm model is serializable, it's JSON can be sent via REST API so the registry can validate a package without having to download all dependencies and build the model in the backend. API for deleting package. registry#9

• serialization/deserialization should change none of the contents of the fpm_model_t (i.e., path variables should not change) but just provide an automated way to load it back/forth

[perazz]

This has been merged, so I will close this issue.