[Ingest Manager] Discuss: Add route for installed packages

[jfsiii]

Problem

There isn't an API to list/query/filter installed (epm-packages) packages. /epm/packages maps to the available packages from the Registry; not installed ones.

So, for example, when the UI for listing the installed packages calls /epm/packages we must iterate/filter a list of all packages on the registry instead of querying ES for those which are installed.

This also means we must implement any search & filtering for installed packages in JS instead of using (or starting with) ES.

Some options

1. Add options to existing /packages endpoint e.g. /epm/packages?status=installed&etc=etc or /epm/packages/installed?etc=etc
2. Add one new route for the saved objects e.g. /epm/installed, or /epm/saved_objects
3. Add two new routes e.g. /epm/packages/available & /epm/packages/installed

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)