Openstore returns null if CertificateIdentifier is empty

[mregen]

Proposed changes

August testing:

• OpenStore behavior changed to return null if the storepath or storetype are invalid or empty, which is a change to the previous release. Hence some support functions should check the return value of OpenStore.
• Fix: Check for null in LoadPrivateKeyEx
• in addition: try fix the new flaky test issue
• fix #2637: an exception when the https server starts and a X509Store certificate has a non exportable private key, it is not needed to create a copy for the TLS transport. Ignore the error and start the cert with the original cert.
• restructure which platforms are supported by https and complextypes, to avoid dependency collisions. Remove netcoreapp3.1 (eol), support netstandard2.0/2.1 instead.

Related Issues

• Fixes #2637

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.

Project coverage is 55.05%. Comparing base (41363bd) to head (c767d35). Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
...ndings.Https/Stack/Https/HttpsTransportListener.cs	57.14%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2761 +/- ## ========================================== - Coverage 55.16% 55.05% -0.12% ========================================== Files 349 349 Lines 65694 65697 +3 Branches 13440 13441 +1 ========================================== - Hits 36239 36167 -72 - Misses 25618 25692 +74 - Partials 3837 3838 +1 ```

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