Add cloud functions e2e test

[dashpole]

This is split into 3 commits:

Fix detector

The detector was incorrectly detecting the cloud run platform instead of the cloud functions platform. This is because cloud functions v2 is implemented on top of cloud run. This commit makes the detector check for cloud functions first to fix this issue.

Reorganize e2e-test-server

This commit splits the scenario handler into its own folder. This way, environment variables like SUBSCRIPTION_MODE are only checked when using the push/pull server, and don't cause cloud functions tests to fail.

Add cloud functions test

A few problems I ran into while getting this to work:

• The function entrypoint must be at the root of the "project". This means we can't just zip up the entire repo and use it as our function.
• If we zip up the cloud functions directory as-is, it will try to fetch dependencies from the internet, including other modules in this repo (like the trace exporter!). It doesn't have access to or understand replace statements.

To solve these, we go mod vendor the dependencies into the same directory as our cloud function, which copies the files from elsewhere in our repo. We also zip -r to ensure the vendor directory is also zipped.

[codecov]

Codecov Report

Merging #552 (60c68d1) into main (bfb214a) will decrease coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #552      +/-   ##
==========================================
- Coverage   64.19%   64.00%   -0.20%     
==========================================
  Files          37       37              
  Lines        4175     4175              
==========================================
- Hits         2680     2672       -8     
- Misses       1382     1390       +8     
  Partials      113      113              

Impacted Files	Coverage Δ
detectors/gcp/detector.go	76.19% <100.00%> (ø)
...lector/internal/datapointstorage/datapointcache.go	80.30% <0.00%> (-6.07%)	⬇️

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