add build policy validation

[kartikp10]

Description

Adds support for validation of code security build policies.

Motivation and Context

This update was to ensure that code security build policies can be validated before creation in the terraform provider.

How Has This Been Tested?

This was tested by creating code security build policies using the terraform provider. The following tests were conducted to ensure existing functionality is not affected:

• creation of a run-only policy
• creation of a valid build-only policy
• creation of an invalid build-only policy
• creation of a run + 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.