Support iterables in adapt() for preprocessing layers (#22417)

* feat(preprocessing): support iterables in adapt() for Discretization, IndexLookup, and TextVectorization

* fix(preprocessing): address review feedback for iterable adapt support

* fix(tests): use correct Grain API for adapt tests

* fix(tests): return raw values from Grain source, not wrapped lists

* fix(tests): move grain import to top level, remove try/except guards