refactor(rds_custom toolset): Group toolsets by read/write intent

[careyson]

refactor(rds_custom toolset): Group toolsets by read/write intent

This refactors the toolset grouping logic for the RDS Custom product to provide a clearer and more intuitive model for users.

• rds_custom_read: read-only toolset for safe auditing, monitoring, and analysis. It contains all Describe* class interfaces.

• rds_custom_action: write-enabled toolset for all state-changing operations (e.g., Create*, Modify*, Delete*). It must be loaded alongside rds_custom_read to be effective.

• rds_custom_all: meta-group that conveniently loads both read and action toolsets for full management access.