Additional changes for v7 compatibility

[Shane32]

This PR targets bump / PR #254

• Use ExecutionOptions.User for ClaimsPrincipal if user context does not implement IProvideClaimsPrincipal
• Asynchronous execution of policies, eliminating all calls to GetAwaiter().GetResult()
• No API changes
• Add note to readme regarding unsupported features and ASP.NET Core compatibility
• Separate format check from tests
• Add note that this implementation has a critical security flaw

There's really very little code changes; mostly just refactoring.

Tests run locally but don't run because this PR doesn't target master or develop.

Hide whitespace to reduce the diff

[Shane32]

@sungam3r Feel free to merge into your PR and continue to make changes as desired.

[sungam3r]

Thanks.

Use ExecutionOptions.User for ClaimsPrincipal if user context does not implement IProvideClaimsPrincipal

I would like to remove IProvideClaimsPrincipal at all. I see no sense to duplicate API that was added in the root lib.