Fix a group of bugs in plugin SQL API

Summary

• feat: test plugin sql api with pgproto

• fix: create plugin for non-superuser

Plugin installation doesn't pass through a regular raft pipeline for DDL commands. As a result, we didn't check superuser privileges for the user. And it could case panic on an attempt to write to the raft log (the regular user doesn't have permission for it).

• fix: plugin SQL via pgproto

• fix: sbroad panic on drop plugin parsing

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• Depends on: sbroad!528 (merged)
• Close #945 (closed)
• Cherry-pick to: none
• Docs follow-up: not necessary