Synthetic latency tests

[yuki-stripe]

Summary

• Adds MPELatencyTest, test suite that measures PaymentSheet.load under various configurations.
  • This test is ignored in StripePaymentSheet / AllStripeFrameworks
  • It reports load times via a mpe.synthetic_latency analytic and by printing SYNTHETIC_LATENCY_RESULT (which will be picked up by a benchmark script later).
• Adds StripePaymentSheet-LatencyTests scheme for ^
• Adds bitrise job latency-tests that runs ^

Motivation

https://docs.google.com/document/d/1nQdXDF4tzWEWCSGcy0XEq6pzDl6NuHz_0_8O3soCkj0/edit?tab=t.0

Testing

Have been manually testing bitrise job -> hubble pipeline.

Changelog

Not user facing.

[github-actions]

🚨 New dead code detected in this PR:

CheckoutSessionConfirmResponse.swift:13 warning: Unused class 'CheckoutSessionConfirmResponse'
CheckoutSessionConfirmResponse.swift:52 warning: Unused extension 'CheckoutSessionConfirmResponse'
STPAPIClient+CheckoutSession.swift:63 warning: Unused function 'confirmCheckoutSession(sessionId:paymentMethod:expectedAmount:expectedPaymentMethodType:returnURL:shipping:clientAttributionMetadata:passiveCaptchaToken:)'
STPTestingAPIClient+Swift.swift:198 warning: Unused function 'fetchCheckoutSession(merchantCountry:)'

Please remove the dead code before merging.

If this is intentional, you can bypass this check by adding the label skip dead code check to this PR.

ℹ️ If this comment appears to be left in error, double check that the flagged code is actually used and/or make sure your branch is up-to-date with master.

[find-dead-code]