Enable agent path template customization for azure_msi node attestor plugin

[guilhermocc]

Pull Request check list

• [x] Commit conforms to CONTRIBUTING.md?
• [x] Proper tests/regressions included?
• [x] Documentation updated?

Affected functionality

• Added the ability to use a custom agent path template for azure_msi server node attestor plugin.

Description of change

• Keep the actual azure_msi agent SPIFFE ID form as the default to maintain compatibility
• azure_msi plugin now accepts a new optional configuration agent_path_template that is used for building the agent's SPIFFE ID.
• Added missing documentation for gcp_iit plugin

Which issue this PR fixes fixes #3420

[azdagron]

Thanks for opening this, @guilhermocc ! I'll see if I can get it reviewed here shortly. Just for information, you don't need to bother keeping the branch up to date. Once it has been approved, we (the maintainers) will update it as needed.