Release and CI automation

[Perdjesk]

Goals:

• CI can build and sign artifacts
• CI can publish artifacts to SVN and Nexus
  • SVN dev: https://dist.apache.org/repos/dist/dev/incubator/baremaps/
  • Nexus staging: https://repository.apache.org
• CI triggers build, sign and publish on git tags
• CI generate release candidate vote email and release announcement email
• Reproducible published artifacts (i.e published artifacts with checksum can be reproduced by any machine with exactly the same checksum)
• Automation can be executed by committers on their own machine with their own credentials or signing key.

Relates to: #492

• SVN and Nexus technical baremaps credentials
• GPG Signing key configured in incubator-baremaps repo as GPG_SECRET_KEY
• Reproducible artifacts

References:

• https://infra.apache.org/release-signing.html#automated-release-signing
• https://issues.apache.org/jira/browse/INFRA-23996

[bchapuis]

I just logged an issue on INFRA to setup the secrets on github.
https://issues.apache.org/jira/browse/INFRA-25610

And drafted a PR for the automation of the release process.
#844