Fix issues on our Swift Package Index listing

[jbelkins]

Describe the issue

With the completion of #505, aws-sdk-swift is now listed on the Swift Package Index. I submitted the package for automatic indexing, and it is listed here.

However, our build is shown as broken for all platforms and Swift versions. From the SPI maintainer:

We're having some build machine issues at the moment (nothing to do with this package being added!) and so the build results aren't coming out quite right at the moment. I do believe that for your linux builds you'll need to use our base image that includes openssl via the .spi.yml file (more information here). The build failures for the other platforms may be our issue, though. We're looking into it.

Create a config file as described in the link, and perform any other config changes as needed to show correct build status on Swift Package Index.

Links

https://swiftpackageindex.com/swiftpackageindex/spimanifest/0.12.0/documentation/spimanifest/commonusecases#Images-for-Linux

[jbelkins]

Reopening while I verify that SPI builds are fixed