[NUnit] Fix async behaviour in [AllureStep] aspect + Allow '{obj.prop}' placeholders in [AllureStep] step name

[overlord]

This pull request:

1. additionally fixes race conditions in async part of NUnit AllureStep aspect (it's the continuation of PR Add support for async NUnit tests #314),

2. provides the ability to use "{obj.prop}" placeholders in [AllureStep] step name attribute.

[AllureStep("Login with credentials: {user.Name} / {user.Password}.")]
public void LoginWith(User user)
{
    ...
}

This approach can still be combined with index-based placeholders

[AllureStep("Check if user has role; RoleId: {0}; UserName: {user.Name}.")]
public void CheckRole(int roleId, User user)
{
    ...
}

I've provided some unit tests for this (see AllureStepNameInternalsTest)

I've tried to make this PR to main-branch, but it seems like 2.9.4-preview branch is way more up to date.

[overlord]

Hello, @neparij!
I've fixed some inconsistent behaviour near task continuation with ContinueWith in AllureStepAspect.
My previous PR doesn't cover all faults in async cases.

[neparij]

@overlord thank you! I'll take a look soon.