Index patterns api - server support for loading field list

[mattkime]

Index patterns api for the server lacks field list loading functionality. This is because rollup index patterns and normal index patterns have different methods of generating field lists and the methods are accessed via different REST API endpoints.

• Refactor existing field list loading code so the rollup version is a modification of the general case, looked up via registry
• Make functionality available via REST and server side APIs

Note - there's custom behavior for rollup index patterns that we should be able to remove when elastic/elasticsearch#58339 is merged. The larger PR - elastic/elasticsearch#42720. We'll likely want to remove the custom rollup code as a first step.

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)