Feat: Add an --explain flag to sqlmesh plan to list out plain steps that will happen in finer detail.

[sungchun12]

I met with the SELECT TOP group and SQLMesh can feel opaque at times in how it operates. This is especially true with destructive vs. non-destructive changes to incremental models. Further complicating this is --forward-only plans. Before a user makes a complicated change, it's helpful to get a logical explanation of what SQL operations will take place.

Should be easy at a glance to verify that the potential SQL operations are behaving as expected. For example, I want to make sure a forward only change to an incremental model will drop or change the data type of a column properly before I apply the plan.

sqlmesh plan dev --explain
sqlmesh plan dev --forward-only --explain

(.venv) ➜  sqlmesh-demos git:(sung/vscode) ✗ sqlmesh plan dev --forward-only  --explain    


Differences from the `dev` environment:

Models:
└── Directly Modified:
    └── demo__dev.customers

---                                                                                                                                                          
                                                                                                                                                             
+++                                                                                                                                                          
                                                                                                                                                             
@@ -65,4 +65,4 @@                                                                                                                                            
                                                                                                                                                             
 SELECT                                                                                                                                                      
   *                                                                                                                                                         
 FROM final                                                                                                                                                  
-LIMIT 5                                                                                                                                                     
+LIMIT 10                                                                                                                                                    

Directly Modified: demo__dev.customers (Forward-only)
   Will apply this new SQL model definition next time this runs on a schedule
Apply - Virtual Update [y/n]: 

The look and feel is similar to these plainer term steps in the full walkthrough: https://sqlmesh.readthedocs.io/en/latest/examples/incremental_time_full_walkthrough/#making-changes

Internal slack thread discussing this