Build policy validation

[kartikp10]

Description

Adds logic for validating code security build policies before creation. This PR directly depends on this update to the SDK PaloAltoNetworks/prisma-cloud-go#40

Motivation and Context

YAML build policy strings should be validated before their creation to avoid unexpected behavior.

This PR adds 3 checks and diagnostics for helping the user fix the issues:

1. Validates if the policy string is valid YAML
2. Validates the policy.Rule.Children.Metadata object structure
3. Validates the definition of the policy against the API (https://prisma.pan.dev/api/cloud/code/policies#operation/validateCustomPolicy)

How Has This Been Tested?

The following tests were conducted:

• Creation of a run-only policy
• Creation of a build-only policy
• Creation of a run + build policy
• Creation of an invalid build policy

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes if appropriate.
• All new and existing tests passed.