Expose Kopf's version in user-agent & in logs

[nolar]

Show the version in logs on startup:

[2021-05-14 15:06:23,079] kopf._core.reactor.r [DEBUG   ] Starting Kopf 1.32rc1.
[2021-05-14 15:06:23,080] kopf._core.engines.a [INFO    ] Initial authentication has been initiated.
[2021-05-14 15:06:23,080] kopf.activities.auth [DEBUG   ] Activity 'login_via_pykube' is invoked.
………

And in the API request as User-Agent:

User-Agent: kopf/1.32rc1

If a dev-mode version is installed, the version will look like this:

[2021-05-14 15:08:30,213] kopf._core.reactor.r [DEBUG   ] Starting Kopf 1.32rc2.dev22+ga97ef9eb.

All in all, the version is retrieved from the virtualenv — as python/pip/pkg_resources see it. The source code does not store the hard-coded version anywhere.