Add model building and generate maven publications

[haydenbaker]

Describe your changes

• Adds gradle for building model artifacts (smithy model JARs)
  • Uses smithy-jar to package up models into model jars
• Subprojects are dynamically created for each model directory
• Versions and registration of said projects are managed via properties file
  • These versions should be managed using the update_versions script, where a set of updated services is passed in to update the respective version properties
  • Versions are independent of eachother, meaning that a version should only be bumped when that project changes (and the "all" version is essentially always bumped)
  • Versions are always bumped patch-wise, so they will always be 1.0.X
• An "all" publication is created, it is a platform project that depends on all the subprojects
  • Meaning its pom has a dependency on each model
• Adds .gitignore

Staging

• Here is an example of staging (selective): staging.zip
  • This was created via gradle :xray:publish :acm:publish :publish

Follow-up

• Add github action for CI
• Incorporate JReleaser for publishing

Checklist

• I have read the Contributing Guidelines.
• I confirm that this pull request is not against the generated models.
• My contribution is licensed under the Apache-2.0 license.