Add observer for secure connection info

[rhysparry]

Background

We want to be able to track the TLS protocol versions used in secure connections. This pull request adds an ISecureConnectionObserver which can be used to determine the protocol used for incoming and outgoing secure TCP connections.

Results

When secure connections are established (and authorized), a call will be made to the observer providing key details of the connection:

• Whether the connection is incoming or outgoing
• The thumbprint associated with the connection
• The TLS protocol used by the connection

How to review this PR

Quality ✔️

Pre-requisites

• I have read How we use GitHub Issues for help deciding when and where it's appropriate to make an issue.
• I have considered informing or consulting the right people, according to the ownership map.
• I have considered appropriate testing for my change.

[LukeButters]

Consider adding a test.