✨ Add Json Schema Generation

[NotJeremyLiu]

This PR:

• Adds schema gen files to all contracts
• Adds a make command to generate entry point schema (only entry point is necessary since that is the only contract callable by external parties)
• Adds a CI check to ensure the schema is up to date given any changes

NOTE: Tried to use https://github.com/omissis/go-jsonschema to generate go types from the json schema, but it only generates the types themselves and not the Msgs. For example, there is no ExecuteMsg enum generated into go code. So for now having the json schemas will have to do.