Add ctx usage in Client4 methods

[pmokeev]

Summary

Replaced http.NewRequest method in doUploadFile and uploadPlugin functions by http.NewRequestWithContext method, so now it uses context variable.

Release Note

NONE

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[pmokeev]

/check-cla

[pmokeev]

/release-note-none

[pmokeev]

@hmhealey @devinbinnie can you have a look at this PR?

[hmhealey]

Thanks for the PR! I'm actually going to reassign the reviewers on this to people who are more familiar with the server-side code

[agnivade]

Ben was driving the change around adding contexts. Will defer to him.

[agnivade]

It seems like there are other places as well where we can apply this change. Let's do all of them at once.