Add plugin base (#1)

* [WIP] Adding baseline of plugin

Currently, this plugin is a rough draft of the end product I am aiming for, with additional functions to be added.

For the time being, it adds support for acr/dockerhub/ecr/gcr but has not been tested. This will need to be tested on each provider.

On next commits I will add Buildkite as a provider, as well as potentially package cloud, as it's more or less a copy and paste job with the way I've designed this to be reusable for the most-part and providing our solutions here is logical for an in-house plugin.

I will move on to testing functionality to ensure the base foundations are working as expected and make changes from there to get it more in line with my vision.

[WIP]

* Fix unbound vars

Fix plugin prefix

Fix PLUGIN_PREFIX maybe

wat

* Fix Google Artifact Registry

Fixes
```
name invalid: Missing image name. Pushes should be of the form docker push HOST-NAME/PROJECT-ID/REPOSITORY/IMAGE
[ERROR]: Failed to push cache image
[ERROR]: Failed to save cache to GCR
```

Also, rename gcr to gar as GCR is gone:
https://cloud.google.com/artifact-registry/docs/transition/transition-from-gcr

* Removing acr/dockerhub

* Adding debug verbosity

* Removing cruft

refactor: remove multi-registry support in favor of single-registry calls

I had initially intended for the plugin to allow multiple calls to different registries, but decided it was too convoluted and would be best for multiple calls to the plugin within a pipeline for this functionality

* Cleaning up duplications

* Feat: consolidate hooks and enhance caching functionality

- Remove redundant command and post-command hooks
- Consolidate caching logic into pre-command hook with Docker build support
- Add support for multiple caching strategies (artifact, build-time, hybrid)
- Implement complete provider architecture for ECR and GAR registries
- Add configuration validation and dependency checking
- Expand plugin.yml with full feature set including build args, secrets, and strategies
- Remove unused PLUGIN_PREFIX variable and standardize environment variable usage
- Add automatic cache key generation from dependency files
- Implemented cache hit/miss logic with fallback strategies

* -n is missing on MacOS?

* Expansion issue with -u set

* Adding var to placeholder

* Adding fixes for finding cache

- Will look for :latest tag when it cannot find cache key (in the case that it was auto generated)
- Fixed gar.io, I think this was assumed as it's a GoDaddy parked domain and Google look to have moved to pkg.dev

* Rework hash on Dockerfile

* Update pre-command

* Adding redundancy checks

* Rename "build-time" as it sounds clunky

* Adding bat tests (working)

- Added some comments here as had to create functions to work around some stub issues.

* README, linting

* Lint and spellcheck fixes

---------

Co-authored-by: Joe Coleman <joe@buildkite.com>