feat(aws): build aws sdk

[andydunstall]

Builds the AWS SDK (not yet integrated)

The SDK recommends using find_package, though since we're using add_third_party I don't think that will work. So instead this just adds the libraries outputted when running find_package locally. Not sure if theres a nicer way to do this

(Note I compared the s3v2_demo size from https://github.com/romange/helio/pull/141 with s3_demo and they were about the same, so I don't think adding the SDK increases the binary size much)

Running locally building s3_demo vs s3v2_demo - this does increase build time from around 1 minute to 3 minutes (on a fresh build so not cached)... Unfortunately there's lots of dependencies we only use a small part of but AWS still builds, which we can't really avoid without relatively big changes to the SDK

[codecov-commenter]

Codecov Report

Patch coverage: 94.44% and project coverage change: +1.30% :tada:

Comparison is base (f7d663e) 76.28% compared to head (d74b92f) 77.59%. Report is 2 commits behind head on master.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #142 +/- ## ========================================== + Coverage 76.28% 77.59% +1.30% ========================================== Files 98 98 Lines 7123 7136 +13 ========================================== + Hits 5434 5537 +103 + Misses 1689 1599 -90 ``` | [Files Changed](https://app.codecov.io/gh/romange/helio/pull/142?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman) | Coverage Δ | | |---|---|---| | [util/fibers/epoll\_proactor.h](https://app.codecov.io/gh/romange/helio/pull/142?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman#diff-dXRpbC9maWJlcnMvZXBvbGxfcHJvYWN0b3IuaA==) | `100.00% <ø> (ø)` | | | [util/fibers/proactor\_base.h](https://app.codecov.io/gh/romange/helio/pull/142?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman#diff-dXRpbC9maWJlcnMvcHJvYWN0b3JfYmFzZS5o) | `92.53% <ø> (ø)` | | | [util/fibers/epoll\_proactor.cc](https://app.codecov.io/gh/romange/helio/pull/142?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman#diff-dXRpbC9maWJlcnMvZXBvbGxfcHJvYWN0b3IuY2M=) | `87.73% <66.66%> (+11.06%)` | :arrow_up: | | [util/fibers/epoll\_socket.cc](https://app.codecov.io/gh/romange/helio/pull/142?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman#diff-dXRpbC9maWJlcnMvZXBvbGxfc29ja2V0LmNj) | `73.55% <100.00%> (+0.25%)` | :arrow_up: | | [util/fibers/fibers\_test.cc](https://app.codecov.io/gh/romange/helio/pull/142?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman#diff-dXRpbC9maWJlcnMvZmliZXJzX3Rlc3QuY2M=) | `99.10% <100.00%> (+0.01%)` | :arrow_up: | | [util/fibers/uring\_socket.cc](https://app.codecov.io/gh/romange/helio/pull/142?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman#diff-dXRpbC9maWJlcnMvdXJpbmdfc29ja2V0LmNj) | `75.55% <100.00%> (+0.21%)` | :arrow_up: | ... and [3 files with indirect coverage changes](https://app.codecov.io/gh/romange/helio/pull/142/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Roman+Gershman)

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