AssertingSorted(Set|Numeric)DocValues should be unwrappable [LUCENE-9357]

[asfimport]

1. Obviously singular docValues might mimic multivalued ones via DocValues.singleton(). However, some algorithms prefers to DocValues.unwrap() them if possible.
2. AssertingDocValues blocks this unwrapping slightly changing codepath for singular DVs.

   AS IS

   AssertingDV -> Singleton -> SingularDV

   TODO

   Singleton -> AssertingDV -> SingularDV

I think it's trivial, worthwhile and 0% risk. Are there any concerns?

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Migrated from LUCENE-9357 by Mikhail Khludnev (@mkhludnev), resolved Dec 18 2021
Parent: #10368

[asfimport]

Adrien Grand (@jpountz) (migrated from JIRA)

+1 to this proposal

[asfimport]

Mikhail Khludnev (@mkhludnev) (migrated from JIRA)

will be addressed in enclosing issue