Introduce Ci::JobDefinition model

Task

Based on the Draft: PoC: split build metadata into `job_prot... (!193948 - closed) we need to create a new table p_ci_job_definitions and relative model Ci::JobDefinition

• Create table p_ci_job_definitions
• Create a table p_ci_job_definition_instances to be the association table between definitions and ci_builds.
• Create Ci::JobDefinition model (not yet used in the pipeline creation)
• Add JSON schema validation for config column (reuse existing schema for id_tokens and secrets) - TBD in Validate ci_job_definitions.config with JSON sc... (#560157)