options for content.Writers; options to pre-provide input and output hashing, and blocksize

[deitch]

In most cases, e.g. a remote OCI registry store, you want to hash the retrieved object and validate it. But sometimes you don't, e.g. if you truly trust the store, or if the trade-off in hashing cost is too high. This gives you the option to skip hashing.

[deitch]

@jdolitsky as discussed. I have no idea why CI is failing, looks like some pulls from docker hub?

[deitch]

I restructured this to have the options be variadic functions, so the API is unchanged. Very similar to how https://godoc.org/github.com/containerd/containerd/content#WriterOpt works.

[deitch]

@jdolitsky I completely restructured this. I realized that the main reason to suppress hashes is likely because you already have them. Also, perhaps more importantly, you sometimes need those hashes for the underlying store, e.g. MemoryStore or FileStore.

So instead of an option to SkipHash, this has an option to provide the hash. If the hash is provided, it skips calculations. If you don't care what it is, provide the fixes BlankHash.

This also enabled me to set up some tests on it.

[deitch]

Done. Back at you @jdolitsky

[jdolitsky]

👍