Add unit tests for activator

[tanzeeb]

Refactored retryRoundTripper and activationHandler into multiple
components to make testing simpler.

Fixes #1231

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[mattmoor]

/hold

Wait until after we cut first release.

[tanzeeb]

/hold cancel

[tanzeeb]

/assign @josephburnett

[mdemirhan]

@tanzeeb - I didn't see this PR before and I was doing the exact same work on my side - Luckily I was half complete.

Can you take a look at my own version of round tripper refactoring here:
https://github.com/mdemirhan/serving/blob/activatortraces/pkg/activator/httptransport.go
https://github.com/mdemirhan/serving/blob/activatortraces/pkg/activator/httptransport_test.go

I have a biased preference towards using the implementation above for a couple of reasons :-)

• It instantiates the transport for HTTP2 only once and keeps reusing it. Creating an HTTP2 transport for every transaction is unnecessary
• It implements the RoundTripper interface on the pointer rather than the value preventing extra memory copies.

Please take a look and if it looks good, feel free to copy it over this one as part of your PR. I will wait for your changes to get in before I add tracing features to activator.

Let me know what you think.

[tanzeeb]

@mdemirhan awesome, I'll update the PR!

[tanzeeb]

@mdemirhan I've posted updates in #1689 , please let me know what you think.

[tanzeeb]

Continuing this PR on #1689, since the original fork is a closed source repo.