Support releasing multiple SDKs

[daymxn]

Per b/330880975,

This updates our release infrastructure to support our new multi project repo (with common and generativeai living next to one another). Now, we can release either in separation of the other.

This PR also includes the necessary changes to various tasks, plugins, workflows, tools, and even documentation.

More specifically, this PR fixes the following:

• b/332890790 -> Implement a task to generate collective release notes
• b/332891281 -> Implement a task to generative collective api changes
• b/332891563 -> Provide a means for SDKs to generate their release artifacts in isolation of one another
• b/332892199 -> Separate api changes according to sdk
• b/332892499 -> Implement a task to release all sdks with changes
• b/332893058 -> Update the api changes workflow to work for multiple sdks
• b/332894984 -> Update the build release workflow to work for multiple sdks
• b/332897987 -> Update DEVELOPING to reflect the multiple sdk structure
• b/332898189 -> Update change to allow specifying projects