feat: Add support for nulls_distinct in custom indexes #1151

Job	Run time
ash-ci / mix deps.get	14s
ash-ci / mix hex.audit	22s
ash-ci / MIX_ENV=test mix.compile	48s
ash-ci / MIX_ENV=dev mix.compile	42s
ash-ci / mix spark.cheat_sheets --check	1m 0s
ash-ci / mix spark.formatter --check	52s
ash-ci / mix dialyzer	7m 20s
ash-ci / mix credo --strict	52s
ash-ci / mix format --check-formatted	48s
ash-ci / mix sobelow	54s
ash-ci / mix test	1m 10s
ash-ci / mix deps.unlock --check-unused	43s
ash-ci / Release	0s
	15m 45s

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