Add `log4j-api` to Log4j Core 3.x binary distribution

[ppkarwasz]

Since the refactoring of Log4j Core 3 on top of Log4j API 2 in version 3.0.0-beta3, the binary distribution published on our website does not contain log4j-api any more.

To be honest, I don't entirely understand the utility of the binary distribution, but if we are going to publish one, log4j-api should probably be included in it.

[vy]

In 3, shall we experiment with not providing any binary distributions and instead share instructions on how users can download themselves from Nexus? This will enforce users to consult the installation guide, and hopefully, use log4j-bom instead, which is a win-win.

[ppkarwasz]

In 3, shall we experiment with not providing any binary distributions

Sounds good to me.