Prepare for Sonaype release

[aSemy]

add configuration required for Sonatype releases (signing, Javadoc jar, sources jar)

Signing artifacts is absolutely the worst part of Gradle I've seen. The provided plugins are all out of date, and can cause massive headaches. I've bashed my head against that wall many times, so hopefully this configuration works well.

I've not been able to run it yet because I don't have the signing tokens setup on my current machine, but here are the basic steps:

1. Create a GPG key https://central.sonatype.org/publish/requirements/gpg/#why-do-we-have-requirements
2. in $GRADLE_USER_HOME/gradle.properties, store the relevant properties

   signingKeyId=...
   signingPassword=...
   signingSecretKeyRingFile=...

3. Create a Sonatype Jira account, and again store the user/pass in $GRADLE_USER_HOME/gradle.properties

   ossrhUsername=...
   ossrhPassword=...

4. try running ./gradlew publishAllPublicationsToMavenProjectLocalRepository and seeing if the required .asc files are created

[sanity]

Let's see, I could create a GPG key and store the private key as a Github Actions secret? The sonatype username/password is already stored in MAVEN_PASSWORD and MAVEN_USERNAME.

[aSemy]

First I'd try it locally, then try setting it up in GitHub.

Because the properties come from providers.gradleProperty(), the properties can be defined either in a gradle.properties file, or as an environment variable prefixed with ORG_GRADLE_PROJECT_. So In GitHub I'd rename the environment variables to match this format.