Add action2idl script

[Greek64]

Though the rosidl_adapter has scripts to generate IDL files from .msg and .srv files, a script for .action files is missing.
This PR rectifies that.

[hidmic]

@Greek64 LGTM overall, but I will point out that in Galactic and onwards, you can also do rosidl translate --to idl path/to/my.action

[Greek64]

@Greek64 LGTM overall, but I will point out that in Galactic and onwards, you can also do rosidl translate --to idl path/to/my.action

I didn't know that, thanks.
Is that documented anywhere?

[hidmic]

Is that documented anywhere?

ros2/design#310 is probably the best place. Arguably, it should be replicated here.

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CI up to rosidl_adapter:

• Linux
• Linux-aarch64
• Windows

[hidmic]

@Greek64 mind to alphabetise module import order? That should please flake8.

[Greek64]

@hidmic Done.

[sloretz]

CI (repos file build: --packages-up-to rosidl_adapter test: --packages-select rosidl_adapter)

• Linux
• Linux-aarch64
• Windows

[sloretz]

LGTM with green CI

[sloretz]

Thanks for the PR!