Implements option persisters for feature flags and replaces calls to IExperimentationService with reads/writes of the corresponding options.
As with other options, feature flag options defined in Workspaces and Features layers are serialized and available OOP via Solution.Options. All other feature flag options are only available in-proc. This means the feature flag options need to be defined along the feature implementation, not in central location like WellKnownExperimentNames.

The previous implementation of IExperimentationService in VS layer conflated feature flags and experimental flights.

My understanding is following:
IVsExperimentationService.IsCachedFlightEnabled checks whether the user is selected to participate in a specified flight name. It has nothing to do with feature flags.
IVsFeatureFlags.IsFeatureEnabled checks a feature flag that's stored in VS feature flag storage.

VS provides two services:

1. Control Tower that defines flights.
2. Targeted Notifications that allows us to set feature flags. A specified feature flag can be set for a given flight name (among other criteria).

I believe we should not check flight names directly in order to enable/disable a feature but always use a feature flag. This PR deletes calls to IVsExperimentationService.IsCachedFlightEnabled, which checks for active flight name.

Active feature flags:

Roslyn.AsynchronousQuickActions (used by a/b experiment)
Roslyn.CloudCache (used by a/b experiment)
Roslyn.OOPServerGC (used by a/b experiment)
Roslyn.RemoveUnusedReferences
Roslyn.TypeImportCompletion
Roslyn.TargetTypedCompletionFilter
Roslyn.UnnamedSymbolCompletionDisabled
Roslyn.LSP.Completion (listed in PackageRegistration.pkgdef)
Roslyn.LSP.Editor (listed in PackageRegistration.pkgdef)
Roslyn.ProgressionForceLegacySearch
Roslyn.SourceGeneratorsEnableOpeningInWorkspace
Roslyn.OOPCoreClr (listed in PackageRegistration.pkgdef)
Lsp.PullDiagnostics (listed in PackageRegistration.pkgdef)
Roslyn.PartialLoadMode (listed in FeatureFlags.xml)
Xaml.EnableLspIntelliSense (listed in FeatureFlags.xml)

Only two feature flags are listed in src/vscommon/featureflags/FeatureFlags.xml 4 are listed in PackageRegistration.pkgdef These are displayed in Options > Preview Features. Should we present all feature flags there instead of having some of them displayed in C#/VB specific option panes? Follow up: #55974

Unused feature flags (deleted by this PR):

Roslyn.InsertFullMethodCall
Roslyn.ImportsOnPasteDefaultEnabled
Roslyn.LspTextSyncEnabled
Roslyn.InheritanceMargin

Flight names (Control Tower reports as completed and no longer running, deleted by this PR) - not clear if we still need KeybindingResetDetector at all. Following up.

keybindgoldbarext
keybindgoldbarint

Fixes #43477