Add a Headers preview screen & API End point

[GeekInTheNorth]

For headless builds, the defined headers will need to be accessed via API as part of a NextJS static site build (for example).

An API which provides this data could also be consumed for a new preview tab.

Suggested APIS:

/compiled/list/?pageId={pageId} /compiled/{header}/?pageId={pageId}

[GeekInTheNorth]

Actual APIs created:

• /stott.security.optimizely/api/compiled-headers/list/
• /stott.security.optimizely/api/compiled-headers/list/?pageId={int}
• /stott.security.optimizely/api/compiled-headers/{headerName}
• /stott.security.optimizely/api/compiled-headers/{headerName}?pageId={int}