Newtype OverlayMedium

[stepancheg]

For #11723 OverlayMedium may need a size field. Encoding it in the string will make code too complex, so this PR proposes converting OverlayMedium to a struct.

Even if size should not be added to OverlayMedium, I believe this extra type safety helps, so I'm submitting this PR.

I do not usually write code in Go, so code style feedback is also welcome.

[ayushr2]

Consider adding the optional size to Overlay2 type, instead of complicating the OverlayMedium:

gvisor/runsc/config/config.go

Lines 903 to 909 in d30c58e

	// Overlay2 holds the configuration for setting up overlay filesystems for the
	// container.
	type Overlay2 struct {
	rootMount bool
	subMounts bool
	medium OverlayMedium
	}

The new overlay2 format you are proposing is --overlay2=<which mounts>:<medium>,<options...>. The options will be applied irrespective of the medium (self, memory or anonDir). So we should not pack it as part of the medium.

I think this PR would increase code complexity.

[stepancheg]

OK, closing this PR, let's continue discussion in #11723.