* feat: add allowHeaders to Options
This allows developers to use custom headers in their API requests, and they will be accepted by their mounted app.
* refactor: convert allowCrossDomain to generator to add appId in scope
This is necessary as the middleware may run in OPTIONS request that do not contain the appId within the header.
* chore: update Definitions and docs
* fix: update test to use new allowCrossDomain params
* chore: add tests for allowCustomDomain middleware re: allowHeadrs