Without this, we get some bad behaviors:
* Cannot use React.memo'd components
* Cannot switch between UseField components (causes a "change in the
  order of hooks" error from React)

WIP because: they're probably not treated well in the UI, and they're certainly not
going to execute properly, and there are no tests.

It's mostly just a glorified textarea for now.

* Does not assert the query language, as that is not displayed on Rule
  Details
* Does not exercise rule execution

This is to prevent undefined behavior until EQL execution is
implemented.

I changed the default value for the form field mock from an array to a
string; this fixes the few tests that were relying on it being an array.

I made a pass through our treatment of RuleType to verify that EQL rules
would be treated appropriately. Since the default/fallthrough case is
typically the Query rule, and since this rule has the same
attributes/behavior as the new EQL rule, not much had to change here.

I converted a few if statements to exhaustive switches where possible,
and used predicate helpers in places where it was not.

* master: (38 commits)
  Reporting/Test: unskip non-screenshot tests (elastic#77088)
  Move metrics to setup and add cgroup metrics (elastic#76730)
  [Enterprise Search] Add Overview landing page/plugin (elastic#76734)
  First pass. Change TS type. Update OpenAPI (elastic#76434)
  [CI] Balance xpack ci groups a bit (elastic#77068)
  [Security_solution][Detections] Refactor signal ancestry to allow multiple parents (elastic#76531)
  [Maps] convert MetricsEditor to TS (elastic#76727)
  IndexMigrator: fix non blocking migration wrapper promise rejection (elastic#77018)
  [Enterprise Search] Update config data endpoint to v2 (elastic#76970)
  [ML] Add decision path charts to exploration results table (elastic#73561)
  Bump eventemitter3 from 4.0.0 to 4.0.7 (elastic#77016)
  [Ingest Pipelines] Add descriptions for ingest processors K-S (elastic#76981)
  [Metrics UI] Replace Snapshot API with Metrics API (elastic#76253)
  legacy utils cleanup (elastic#76608)
  [ML] Account for "properties" layer in find_file_structure mappings (elastic#77035)
  fixed typo
  Upgrade to Kea 2.2 (elastic#77047)
  a11y tests on spaces home page including feature control  (elastic#76515)
  [ML] Transforms list: persist pagination through refresh interval (elastic#76786)
  [ML] Replace all use of date_histogram interval with fixed_interval (elastic#76876)
  ...

There was an issue previously where memoized components would not work;
these are primarily regression tests covering that use case.

When swapping between the Custom Query and EQL rule types, we want to
ensure that the corresponding input component coming from UseField fully
unmounts and remounts with the new component.