Add CI workflow to check CDDL syntax

[fxamacker]

Add GitHub Actions (cddl.yml) to check CDDL syntax:

• checkout repo
• install cddl compiler using cargo
• extract CDDL from CCF specifications document and print SHA256 of extracted CDDL
• compile extracted CDDL file

The CCF specification document must have a pair of CDDL comments to mark the CDDL notation to be extracted and compiled:

;CDDL-BEGIN
... 
;CDDL-END 

Closes #14

Thanks @turbolent for suggesting (in PR #10) that we use CI to check CDDL with https://github.com/anweiss/cddl.