Add toggle to run test without downloading embedded resources

[ErikBooijFR]

Currently, by default all tests ran will download all resources embedded in the page. For most cases this is probably a sensible default. However in our case (and this probably isn't really an edge case either) we're serving HTML from our own servers, and serving assets from a CDN.

We want to be able to load test our own service, without accessing the CDN, because:

• It's more representative of the metric we're actually interested in (our servers' performance, not the end-users' total load time).
• We avoid getting rate limited by the CDN, skewing our results.

Description of changes:
This change proposes adding a checkbox labeled "Retrieve embedded resources" under the "Endpoint" field in the "Simple" test type. By default it's checked, so if a user doesn't touch it, the behaviour doesn't change from the current state.

The value of the checkbox is passed to the retrieve-resources property in the generated test definition. See https://gettaurus.org/docs/JMeter/#Global-Settings.

I have not added the checkbox to the form for creating a test from a JMeter test definition file, since it's impossible to tell what's in there and if/how the flag would affect it.

Checklist

• 👋 I have run the unit tests, and all unit tests have passed.
• ⚠️ This pull request might incur a breaking change.

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