Fix PowerShell empty header validation

[cnotin]

Summary

Fixes #108.

• Add conditional SkipHeaderValidation support for requests that intentionally use empty header values.
• Extend the shared Invoke-WebRequest2 helper to handle empty string headers on PowerShell 6+.
• Update direct Azure AD Connect, SharePoint, and Application Proxy REST calls.

Why

PowerShell 6+ validates empty values for standard headers such as User-Agent, causing affected AADInternals commands to fail before sending the request. Windows PowerShell 5.1 does not expose the compatibility switch.

Impact

Empty headers retain their existing on-the-wire intent under PowerShell 6+, while Windows PowerShell 5.1 remains supported through runtime parameter detection.

Validation

• Parsed all edited scripts in PowerShell 7.6 and Windows PowerShell 5.1.
• Verified Invoke-WebRequest2 adds SkipHeaderValidation only when supported and an empty header is present.
• Ran git diff --check.