Add Concurrent Download Support for artifacts

[gowthamgts]

This PR adds support for concurrent downloads as requested on #11244. Currently 3 is set as the max number of downloads that can be active at a time, but I'm happy to add that to client or task configuration.

resolves #11244

[hashicorp-cla]

All committers have signed the CLA.

[gowthamgts]

@lgfa29 @schmichael - can you please take a look and let me know if there are any changes needed?

[schmichael]

Looks good but there should be a test for the case where there are 4 artifacts and the first one fails to download as currently I believe that leaks a goroutine.

A https://pkg.go.dev/net/http/httptest test server should allow testing against an artifact server where you can control the error rate.

[gowthamgts]

hey sorry folks, got caught up with other work, i'll try to get this closed by this weekend.

[schmichael]

Looks great! I pushed a changelog entry. Mind cleaning up the tests a bit more?

I'd like to hold off merging this to main until we do one more 1.2.x release. I'll label it so it doesn't get lost.

[schmichael]

I'll label it so it doesn't get lost.

To be fair, this did work. It didn't get lost! However I did almost forget to merge it for 1.3.0! 😬

Merging now and will be released in 1.3.0-rc1! Thanks @gowthamgts

[github-actions]

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