make it possible to wrap the client transport in another one

[novas0x2a]

When using the client, I'd like to modify the behavior somewhat (retries, context-aware, etc). This would allow me to interject a transport to handle that; I'm not set on this impl, though, and I'd do it differently if you'd prefer that. Putting this up as a feeler.

[GordonTheTurtle]

Please sign your commits following these rules:
/~https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "default-transport" git@github.com:novas0x2a/distribution.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[codecov]

Codecov Report

Merging #2985 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2985      +/-   ##
==========================================
- Coverage   60.69%   60.64%   -0.05%     
==========================================
  Files         102      102              
  Lines        8044     8044              
==========================================
- Hits         4882     4878       -4     
- Misses       2511     2514       +3     
- Partials      651      652       +1

Flag	Coverage Δ
#linux	60.64% <ø> (-0.05%)	⬇️

Impacted Files	Coverage Δ
registry/handlers/app.go	48.16% <0%> (-0.7%)	⬇️

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[caervs]

Hey @novas0x2a. Sorry it has taken us so long to get to this one.

Can you tell me why it's not sufficient to pass in your round tripper as the base for NewTransport? I know that would wrap your roundtripper instead of the other way around, but I think it'd suffice for the cases you bring up.

[novas0x2a]

Sorry, I guess I didn't motivate the change enough; I was looking for a more centralized, less-invasive-and-api-breaking method to add context awareness to things like NewV2Repository which doesn't let me pass a transport in. (I readily agree that my method is not ideal, just not sure what else to do about it, heh).

[caervs]

Fair enough. Works for me. LGTM.

[dmcgowan]

We are trying to deprecate this implementation in favor of the client implementation in containerd. I don't have a better suggestion for how to achieve that in this code base though, I think updating it is fine for now.

LGTM