Add e2e test for strategic merge patch

[roycaihw]

Add a test sending (e.g. config map) patches through the same API using:

• JSON patch
• Strategic merge patch

make sure the API client is able to set the content type properly according to the request body, and the patches are applied successfully.

ref: #931 (comment)

[palnabarun]

I want to give a shot at this. :)

/assign

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[palnabarun]

/remove-lifecycle stale

[roycaihw]

Examples of using these patches:

strategic merge patch: #641 (comment)
json patch: #641 (comment)

We can have one test sending multiple kinds of patch and make sure nothing fails, or two tests doing the same thing but using different kinds of patch.

[fejta-bot]

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