Don't require git to download external projects

[nealrichardson]

Issue #, if available: N/A

Description of changes:

Building aws-sdk-cpp with the default BUILD_DEPS=ON fails if git is not found on the system:

-- Configuring incomplete, errors occurred!

-- stderr output is:
CMake Error at /tmp/RtmpqRJAdR/file1835205980df/cmake-3.18.1-Linux-x86_64/share/cmake-3.18/Modules/ExternalProject.cmake:2350 (message):
  error: could not find git for clone of AwsCCommon
Call Stack (most recent call first):
  /tmp/RtmpqRJAdR/file1835205980df/cmake-3.18.1-Linux-x86_64/share/cmake-3.18/Modules/ExternalProject.cmake:3228 (_ep_add_download_command)
  cmake/BuildAwsCCommon.cmake:39 (ExternalProject_Add)
  CMakeLists.txt:19 (include)


CMake Error at CMakeLists.txt:218 (message):
  Failed to configure third-party libraries.

These third-party cmake files specify a git repository and tag for the source for each library, but we can download the source without git by constructing a URL.

On apache/arrow#8304 I've tested that this works as expected by applying it in a PATCH_COMMAND.

Check all that applies:

• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.) Any CI you have that tests these cmake files will cover the changes; there are no behavior changes
• Checked if this PR is a breaking (APIs have been changed) change.
• Checked if this PR will not introduce cross-platform inconsistent behavior.
• Checked if this PR would require a ReadMe/Wiki update.

Check which platforms you have built SDK on to verify the correctness of this PR.

• Linux
• Windows
• Android
• MacOS
• IOS
• Other Platforms

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[wps132230]

Hi there, we are using git submodule instead of CMake external project now.
If you don't have git on your system, you could run this script to download all submodules: https://github.com/aws/aws-sdk-cpp/blob/main/prefetch_crt_dependency.sh
Does that meet your requests?

[nealrichardson]

I think we worked around this by building the dependencies ourselves rather than relying on BUILD_DEPS=ON