crane: support --omit-digest-tags in crane ls

[imjasonh]

Before:

$ crane ls cgr.dev/chainguard/static | head -n 10
20220302
sha256-bab7d22b93180e8a05d0f02c739605cc7ed52b414b8f70ad6a173f2c0c8dc946.sig
sha256-e86edc0a7f00adb469d13b49192f38f992a981d778aa3f24525aa83b370b4001.sig
20220303
sha256-1535c58eb82f566a382131ba23583135aaba4e22fccccaadcd00b24286c47ad7.sig
sha256-061d30276694f8ae8e6a47eec9e7b7526d5e0c89436217907cac09cc19d9a953.sig
20220304
sha256-6168c94e3afa1b01a090632f77309ce541e1f502d8783071d026eb97a211b723.sig
sha256-8953bc492aebe6574d8735174000a7cea046de6ec8b6b28501ae27110352bc11.sig
20220305
$ crane ls cgr.dev/chainguard/static | wc -l
7890

After:

$ go run ./cmd/crane ls cgr.dev/chainguard/static --omit-digest-tags | head -n 10
20220302
20220303
20220304
20220305
20220306
20220307
20220308
20220309
20220310
20220311
$ go run ./cmd/crane ls cgr.dev/chainguard/static --omit-digest-tags | wc -l     
     369

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