Dev/anasanc/pep addingcommonextensioncode

[alsanmsft]

Changes here implement functions that will be frequently used across different extensions for a feature called Constrained Extensions. Constrained extensions limits the capabilities of onboarded extensions according to a customer-defined, pre-set policy.

The changes in this PR include:

• LoadPolicySettings() to load policy settings into a struct (defined by an extension) from the policy file
• ValidateFileHashInAllowlist() to implement allowlist logic into extensions that run scripts.
• ComputeFileHash()
• ValidateItemInList()
• UTs