Make it easier for people to implement change detection for fine-grained deploys

[huonw]

Is your feature request related to a problem? Please describe.

A common question on slack seems to be: I have multiple artifacts / many libraries / microservices, how can Pants help me identify which ones have changed and deploy only those ones?

For instance:

• 5 different helm charts, and only wants to only run the pants experimental-deploy ... on exactly the ones that have changed.
• 4 python_distributions publish the ones that have changed
• 3 docker_images and only push the ones that have changed
• 2 AWS CloudFormation stacks that depend on resources build in Pants, and only deploy which one has changed

Describe the solution you'd like

Some ideas:

• a documentation "tutorial" similar to https://www.pantsbuild.org/2.19/docs/using-pants/using-pants-in-ci
• an example repository implementing this (combined with the above, potentially)
• making the right approach so self-evident that there's no need to document it 😂

Describe alternatives you've considered
N/A

Additional context

Example threads:

• https://chat.pantsbuild.org/t/16658940/hi-all-i-m-using-pants-for-ci-with-github-actions-and-it-is-#d26dce97-ba25-4193-ae3f-a9a3d8ea06d7
• https://chat.pantsbuild.org/t/16755257/hey-everyone-we-want-to-combine-our-existing-libraries-into-#bfcaa2db-c49e-44b8-bb30-fd65d027c58e

Some issues that are potentially related, in that they may influence how accurate/reliable a solution using --changed-since can be:

• Dependees (dependents) should return dependent files for removed modules #17512
• Pants triggering a rebuild even if dev-dependency updated #15694
• python_distribution plugin conflicts with pants publish --changed-since #19509
• Modifications to __defaults__ in BUILD files are not detected/propagated when using --changed-since #19805

A hash for a target may also be relevant:

• Expose the hash of a given target #8445
• Add sources_fingerprint to peek on source-creating targets #18383 (the recipe there)