Implement release plugin

[daymxn]

Per b/317081045,

This implements a new plugin for preparing a release. This plugin provides functionality to;

• Update the project version declared in the gradle.properties file
• Create a new .api file in the new api directory
• Automatically generate the release notes

This PR also fixes the following:

• b/319482236 -> Store released .api files in a api directory

Warning

This PR should not be merged until the following is complete:

• The branch is not currently in a release state
• All of the previous release's .api files are present in the api directory