Add the missing ssl::context callback in websocket_client_config

[garethsb]

In order to be able to set up certificates. Exactly the same use case as http_client_config and borrowing heavily from http_client implementation.

[garethsb]

@BillyONeal I don't think the reported CI failures are actually related to this PR, am I right?

[BillyONeal]

I think I managed to kick off another try: https://dev.azure.com/vclibs/cpprestsdk/_build/results?buildId=306

Will keep you posted.

[garethsb]

Thanks. It looks like the same two platforms failed:

• Ubuntu 16.04 Vcpkg - which seems to be unable to find Boost?
• Android - which fails building Boost with "Undefined or not supported Android NDK version: 19.0"

[BillyONeal]

Something is borked with Pipelines and vcpkg right now; I reported to vcpkg folks and will keep you posted.

[garethsb]

Thanks. It'd be great to get #1049 and #1051 tested and into master.

[BillyONeal]

Looks like Pipelines is off the floor and your change works. Do want to add a test case for this? (This is small enough that I'll merge it without a test added but we're liable to break the feature on you again without it)

[BillyONeal]

I guess we can always add that test later. Thanks for your contribution!