Add keepalive, set_keepalive to TcpStream implementations

[yungcomputerchair]

What

The current implementation of TcpStream does not expose SO_KEEPALIVE: #69774

Why

It seems the reason this has not yet been implemented is because the initial implementation went as far as to take the keepalive interval as a parameter. However, Windows does not directly expose these intervals for reading, causing there to be some debate about the shape of the API (see the discussion of this here).

How

I am proposing that this API is enabled with a bool parameter to align with the implementation in both Windows and Unix. With regards to the call to setsockopt, Windows expects a disable/enable, and Unix does as well.

The extra configuration for Unix that allows the interval to be tweaked could be done through an additional API perhaps, but I don't think that's stopping us from exposing the switch to at least enable the behavior with the OS defaults.

[rustbot]

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[yungcomputerchair]

As a new contributor, there were a few things I wasn't sure about:

• I don't think there is a "tracking issue" for this feature, just the one I linked above with the discussion. I am guessing a tracking issue will need to be created for this, and the issue number in the code will need to be updated.
• I don't know if unstable features are defined somewhere, so I just made sure the name was unique and matched for both new APIs.
• For motor, the lib does not expose a keepalive setting (according to the docs), so I went with unsupported()
• For SOLID and Hermit, I believe the implementation was straightforward, but I'm not sure how to test. Is there a CI flow I can use?
• Is there anything else I need to do to validate or properly propose this change?

Thanks :)