Build refactor: Move signing and publishing to convention in buildSrc

[JonasKunz]

We currently copy-paste the buildscript configuring the signing and publication (e.g. to inferred-spans). We should use a gradle convention plugin instead.

In general, we should try to remove as much logic as possible from the subprojects {} call in the root project, as this makes the build difficult to understand and is not recommended. We shoult stick to only doing very basic things which are guaranteed to be always the same for all projects, such as assigning the groupid and version.

[SylvainJuge]

As the artifact signature and publishing conventions are only applied in CI when actually publishing to a remote repository, the best way to test any change here is probably to

• open a PR on the upstream repo (not on a fork as we are used to)
• manually trigger the snapshot pipeline on buildkite with the dry_run=true environment variable on the branch we've created (if it's on a fork then buildkite can't build it directly).
• inspect the content of the generated artifacts on buildkite (it should contain an archive that contains signed artifacts that would have been published publicly if dry_run=false).

[jackshirazi]

this should go into a build readme