chore: Inject into hard-coded secondary domain

[chrisbreiding]

• Closes Inject Cypress into a Hard-coded Secondary Domain #16450

Uses the proxy to inject code necessary for Cypress to run into a secondary domain. Currently, it's hard-coded to 127.0.0.1:3501, but that restriction will be removed in the future once other aspects of multidomain are implemented.

This is still an improvement, as before the page being visited had to have the injection code already hard-coded into it, now it's possible to visit any page on 127.0.0.1:3501 and the injection code will be dynamically added.

[cypress-bot]

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[cypress]

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Test summary

17957 • 2 • 217 • 7 • 3

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Run details

Project	cypress
Status	Errored
Commit	af647ac
Started	Jun 14, 2021 7:13 PM
Ended	Jun 14, 2021 7:24 PM
Duration	10:57 💡
OS	Linux Debian - 10.8
Browser	Multiple

View run in Cypress Dashboard ➡️

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Failures

	cypress/integration/commands/net_stubbing_spec.ts		1
1	network stubbing > waiting and aliasing > can timeout waiting on a single request using "alias.request"

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Flakiness

	reporter.errors.spec.js		1
1	errors ui > docs url > displays as button in interactive mode
	retries.ui.spec.js		2
1	runner/cypress retries.ui.spec > opens attempt on each attempt failure for the screenshot, and closes after test passes
2	runner/cypress retries.ui.spec > opens attempt on each attempt failure for the screenshot, and closes after test passes

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[flotwig]

LGTM