Make this a proper package

[schuellerf]

Thanks @croissanne for creating this initially.
Now we need to make a proper package.

• fix the old references like https://github.com/osbuild/splunk_logger/blob/main/go.mod#L1
• introduce github actions
• introduce Makefile? (to be consistent)
• enable .pre-commit-config (pre-commit.com) for faster development

[ondrejbudai]

Please, also copy the LICENSE file. :)

[schuellerf]

... and create a README.md

[schuellerf]

btw - once this is a package and we switched - I'm tempted to add a # Historicals section to both readmes (here and in osbuild-composer) just noting "if you are searching for github.com/osbuild/osbuild-composer/pkg/splunk_logger it was factored out to …
any stong opinions against it?