transition from UUID strings to a full-fledged UUID type

General Summary

Usage of UUID type is inconsistent across database internals.

Expected Behavior

• instance_uuid -> UUID;
• System tables should store UUID strings using UUID type.

Current Behavior

• instance_uuid -> TEXT;
• System tables store UUID strings using TEXT type.

Steps to Reproduce

admin=> EXPLAIN SELECT instance_uuid();
                       QUERY PLAN                        
---------------------------------------------------------
 projection (".proc_instance_uuid"(())::text -> "col_1")
 execution options:
     sql_vdbe_opcode_max = 45000
     sql_motion_row_max = 5000
 buckets = any
(5 rows)

Additional Context

The observed front of work is as follows:

• Make instance_uuid return UUID type;
• System tables should store UUID strings using UUID type;
• Migrating system tables on upgrade must not fail. In addition to the above, I would like to mention that simply changing only the return type of instance_uuid just now is a bad option, since it will break the UX in the sense that now user will have to CAST from UUID to TEXT, since in the system tables we still have TEXT fields where these UUIDs are stored.

Possible Solution

Fixing the return type of the scalar function instance_uuid is quite simple:

  // `sbroad/sbroad-core/src/executor/engine.rs:get_builtin_functions`
  Function::new_volatile(
      get_real_function_name("instance_uuid")
          .expect("shouldn't fail")
          .into(),
-     DerivedType::new(Type::String),
+     DerivedType::new(Type::Uuid),
      false,
  ),

and

  // `sbroad/sbroad-core/src/frontend/sql/type_system.rs:default_type_system`
  Function::new_scalar(
      get_real_function_name("instance_uuid").expect("shouldn't fail"),
      [],
-     Text,
+     Uuid,
  ),

and

  // `src/pgproto/value.rs:PgValue::try_from_rmpv`
+ (Value::String(v), Type::UUID) => {
+     let Some(s) = v.as_str() else {
+         Err(EncodingError::new(format!("couldn't encode string: {v:?}")))?
+     };
+     let Ok(uuid) = Uuid::from_str(s) else {
+         Err(EncodingError::new(format!(
+             "couldn't parse string as valid UUID: {s:?}"
+         )))?
+     };
+     Ok(PgValue::Uuid(uuid))
+ }

Speaking about migrating system tables and their data to a new type - this is a rather non-trivial task that has two solutions:

1. Repack all values from the required fields into a new type "on the fly";
2. Create a new column, copy values with new types into it, clear (via NULL) the previous column, delete the old column, rename the new column to the previous name.