feat: pass parameter overrides as prefixed environment variables

[radekl]

Issue #, if available: -

Description of changes:

I have added a feature that allows users to use action's prefixed environment variables as parameter overrides. The prefix of environment key is removed before passing it to cloudformation.

This approach is based on Heroku Deploy action and addresses couple of issues including:

• a lot of parameters to override
• sensitive inputs are not stored on disk/in the file
• values of parameters passed to action using this way are automatically escaped - no need for additional escape, especially if using Github vars/secrets - whatever github accepts will be passed as parameters.

I have added feature, tests and documentation snippet. Feature should be backward compatible as it's only activated if user adds an envs-prefix-for-parameter-overrides parameter with a desired prefix.

I hope that you will find this feature useful and include it in the main release.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[radekl]

Actually solves some of the already existing issues too:

• ParameterOverrides with commas in them fail #47
• Please update this action to accept parameter-overrides from a YAML file with given format #86 (kind of I would say - envs are defined in YAML format, so... would work?)

[radekl]

@kddejong can I have a little bit of your attention here? 😄

[radekl]

Force pushed all the commits to rebase and update to the changes in tests.

[radekl]

@kddejong Hey, any chances for you to look into it?

[jhult]

@kddejong, any thoughts on this?