Add checking image presence in cache prior to downloading it

[valentinsoe]

This changes allow to use kaniko-warmer multiple times without unnecessary docker image downloads.
To check image presence in cache directory I'm using existing cache function that is used by kaniko-executor.
I've considered building separate function to only check image presence, but it will have pretty much the same code.
Questionable decision is to embed CacheOptions type to KanikoOptions and WarmerOptions. Probably this should be resolved by creating interface providing needed options and implement it both mentioned structs. But I've struggled to get a meaningfull name to it.
To replicate previous behaviour of downloading regardless of cache state I've added --force(-f) option.

This changes provides crucial speed-up when downloading images from remote registry is slow.

Closes #722

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[valentinsoe]

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[sharifelgamal]

Thanks for your contribution!