feat: Add beta Custom Rules endpoints and tools

[Shreda]

[feat] Cloud Posture custom rules endpoints

Description

• Add Custom Rules endpoints.
• Clean up all v1client files to use the more generic client functions where possible.

How to Test

Steps to manually test this PR:

1. git checkout feat_custom_rules_beta.
2. make mcpserver.
3. Move the v1-mcp-server binary onto $PATH.
4. Configure the server with your AI tooling (differs depending on tool used)
5. Then test the following

• cloud_posture_custom_rules_list - List custom rules
• cloud_posture_custom_rule_test - Test a custom rule configuration
• cloud_posture_custom_rule_get - Get a specific custom rule
• cloud_posture_custom_rule_create - Create a custom rule (requires readonly=false)
• cloud_posture_custom_rule_update - Update a custom rule (requires readonly=false)
• cloud_posture_custom_rule_delete - Delete a custom rule (requires readonly=false)

Screenshots / Demos (if applicable)

Checklist

• I have tested my changes locally
• I have updated relevant documentation
• I have assigned reviewers

[trend-iain-durham]

I'm approving as my comments, while are worth investigating could be done in a follow up PR if wanted to get this merged ASAP

[trend-iain-durham]

LGTM