`all_schema()` will skip ExtensionPlan's own schema and fetches schemas from children plans

[waynexia]

Describe the bug

I found a regression when debugging GreptimeTeam/greptimedb#1136. Since #5236 the all_schema() won't use the schema of ExtensionPlan, but its children's schemas.

To Reproduce

Not important

Expected behavior

It should use the schema from ExtensionPlan

Additional context

As @alamb and @jackwener discussed in #5236 (review), all_schema() seems unnecessary. It looks like all the usages of all_schema() are for naming a column (the collected schemas are passed to normalize_with_schemas()). But, only a few plans can have multiple inputs (join, union, and maybe extension). I'm wondering if we can take special care of them inside normalize_with_schemas() and remove all_schema() (using_columns in normalize_with_schemas() looks like something is doing this)