feat: custom cache support

[Farenheith]

This pull request enhances the GitHub Action by introducing support for custom caching actions, providing greater flexibility for users to supply their own caching actions while maintaining compatibility with the existing workflow.

Custom caching support:

• Added new inputs custom-cache-action and custom-pub-cache-action to allow users to specify custom caching scripts for Flutter and pub dependencies. These scripts can override the default caching mechanism.
• Updated the caching steps to use the jenseng/dynamic-uses@v1 action, enabling dynamic selection between the default actions/cache@v4 and any provided custom caching action.
• The provided caching action must be a drop-in replacement of actions/cache.
• Modified the CACHE-HIT and PUB-CACHE-HIT outputs to use fromJSON for parsing cache hit information, as dynamic-uses outputs everything as a single json.

Why?

• Given the new charging policies of GitHub since March 1st for self-hosted runners, any option we can use to decrease pipeline duration is welcome. In this particular case, Flutter bin cache is a big one, and adding it in a custom cache action that uses S3, for example, can decrease transfer costs and be faster than the default GitHub one, which is hosted in their own cloud.

[bartekpacia]

Please update README to explain the changes you've made.

[bartekpacia]

And please also rebase with master to fix the CI, see #387

[Farenheith]

@bartekpacia hi! Thank you for your review. I added an entry in the readme explaining custom cache usability and rebased the branch, too. Can you please take another look?