Maven central publishing

[raubrey-stripe]

Prepare the SDK for distribution. Based off patterns from stripe/stripe-java.

• Removed JitPack (jitpack.yml) and GitHub Packages publishing (this was useful when testing, but not the standard way Stripe distributes java SDKs)
• Added deploy.gradle with Sonatype/Maven Central publishing and GPG signing config (will need to get access to this config for this rep)
• Added gradle.properties with POM metadata (group, artifact, SCM, license, developer info)
• Updated build.gradle to use io.github.gradle-nexus.publish-plugin and signing plugin
• Added .github/workflows/publish.yml that publishes on version tags (v*)
• Added Gradle/Maven installation instructions to README

[raubrey-stripe]

FYI @bensandler-stripe @ksn-stripe