feat: adds action templates and reusable actions

[baywet]

This pull request adds action templates.

fixes #33
fixes #136
fixes #270

closes #238

This is another attempt to solve a scale limitation in the current specification. Action templates are better than the previous parameters proposal because:

• They do not introduce a matrix projection, people may use the template multiple times.
• They do not require "declaring" which environment variables to read in advance.
• They limit the string literal replacement to the templates, which does not impact how actions currently work.
• The template reference mechanism works like OpenAPI references.
• The template resolution mechanism can be isolated as a pre-processing step, and the resolved actions can then be fed to the existing code processing actions.
• It introduces a string literal replacement syntax which can be expanded upon for other sources or used at other places in the future.

The pull request is incomplete as it is, it's a draft, I want to collect feedback on the approach before making any further investments.

[ralfhandl]

I like the general direction.

[baywet]

Feedback from the meeting from both @lornajane and @ralfhandl :

• add a peer block to parameters for the environment variables, this allows to set defaults and "declare" which variables are used. But this declaration is optional.
• add a peer "actions" in the components, that's defined as everything except for the target. So people can specify the target on reference. Reusable actions may use action templates in turns.

[ralfhandl]

Mostly wording and capitalization

[handrews]

The correct way to define this is to say that the value is a same-document (or fragment-only) relative URI reference, per RFC3986 §4.4, and that the fragment syntax is JSON Pointer, with the pointer prefix restricted to /components/actions/.

It looks to me like the reference resolution mechanism here is delegation, as in JSON Schema draft 2020-12, and not replacment, as there is another field in this Object. That should be called out explicitly.