adapter,catalog: convert more builtin item tables into views

[teskje]

This commit converts another set of builtin tables into views over the catalog:

• mz_connections
• mz_continual_tasks
• mz_indexes
• mz_secrets
• mz_sinks
• mz_sources

As part of this, it adds new constant builtin views exposing the respective builtin object kinds to SQL queries:

• mz_builtin_connections
• mz_builtin_continual_tasks
• mz_builtin_indexes
• mz_builtin_sources

Motivation

Part of SQL-118

[github-actions]

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