Standardized HTTP request and response headers

[jpraet]

Reusable components could be added to openapi-common for the HTTP request and response headers standardized in the Belgif guide.

• Caching (https://www.belgif.be/specification/rest/api-guide/#caching)
  • Request headers:
    • If-None-Match
    • (If-Match) not sure, currently not referenced in the Belgif guide
    • (If-Modified-Since) not sure, guide recommends ETag
    • (If-Unmodified-Since) not sure, currently not referenced in the Belgif guide, and guide recommends ETag
  • Response headers:
    • ETag
    • (Last-Modified) not sure, guide recommends ETag
    • Cache-Control
    • Vary
• Other HTTP response headers:
  • Location
  • Retry-After

[pvdbosch]

Agree with the ones not in parentheses. the If-Match probably only after adding a rule on optimistic locking (belgif/rest-guide#37)