Migration conversions

[ldhasson]

Add a new capability to handle more complex migrations that require some expression for a complex type change that cannot be automatically migrated.

For example, let's take the table column definition:

         ,{ "name":"componentType"  , "type":"SHORT"         , "nullable": false, "description": "Type of agentic component called"
                                    , "values": [ { "name":"Tool" , "value":"1" , "description":"Tool"   }
                                                 ,{ "name":"Agent", "value":"2" , "description":"Agent"  }
                                                ]
          }

We want to change to:

         ,{ "name":"componentType"  , "type":"CHAR"          , "nullable": false, "description": "Type of agentic component called"
                                    , "values": [ { "name":"Tool"   , "value":"T", "description":"Tool"   }
                                                 ,{ "name":"Agent"  , "value":"A", "description":"Agent"  }
                                                 ,{ "name":"Unkbown", "value":"X", "description":"Unknown"  }
                                                ]
          }

This is not a migration that can be automated: not only the type change from SHORT to CHAR could lose data (so Tilda wouldn't do that), but we also have semantic changes in value mapping.

We propose to add a new feature to "migrations" to handle that:

 ,"migrations":{
      "conversions":[
         { "object":"AgenticAccessLog", "column":"componentType"   , "conversion":"case when componentType=1 then 'T' when componentType=2 then 'A' else 'X' end" }
       ]
     ,"renames":[
       ]
   }

Tilda will automatically handle dependencies so that if there is a view that depends on that change, either it will fail with a clear message that the view needs to be updated, or Tilda will manage the DDL Dependency tree appropriately.