sqlcmig6: add sqlcmig6 package

This commit introduces the `sqlcmig6` package, which at the time of this
commit contains the same queries and models as `sqlc` package.
Importantly though, once the kvdb to sql migration is made available in
production, the `sqlcmig6` package will not change, as it is intended
to represent the sql db as it was at the time of the migration.

The sqlcmig6 package is therefore intended to be used in the kvdb to sql
migration code, as it is will always be compatible with the sql database
when all sql migrations prior to the kvdb to sql migration are applied.

When additional sql migrations are added in the future, they may effect
the `sqlc` package in such a way that the standard `sqlc` queries and
models aren't compatible with kvdb to sql migration code any longer.

By preserving the `sqlcmig6` package, we ensure that the kvdb to sql
migration code can always use the same queries and models that were
available at the time of the migration, even if the `sqlc` package
changes in the future.

Note that the `sqlcmig6` package have not been generated by `sqlc` (the
queries and models are copied from the `sqlc` package), as it is not
intended to be changed in the future.

Author: ViktorT-11

.golangci.yml

@@ -61,3 +61,9 @@ issues:
         - unused
         - deadcode
         - varcheck
+    # As the db/sqlcmig6 package has been copied from sqlc generated code,
+    # but isn't marked as code generated by sqlc, the code line length in the
+    # package will often exceed the lll limit.
+    - path: db/sqlcmig6/.*
+      linters:
+      - lll