Provide a way to access the SSLSession of a websocket instance

[marci4]

Description

There is currently no good way to access the SSLSession used for a websocket.
Therefore it is not possible to examine client certificates

Related Issue

Fixes #890

Motivation and Context

Clean methods to access the SSLSession.

How Has This Been Tested?

Manual test

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.