Migrate Sts to Smithy for multi auth + add configurable Sigv4aSignerRegionSet

[sbaluja]

Issue #, if available:

Description of changes:

• STS migrates to SmithyClient
• Expose Sigv4aSignerRegionSet as a clientConfig option
• Disable InvokeServiceOperation for SmithyRestXML codegen (it is not generated in Smithy Headers, and is not generated for SmithyJson clients i.e bedrock); we should enable this correctly for smithy in a separate PR

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• Did a review by yourself.
• Added proper tests to cover this PR. (If tests are not applicable, explain.)
• 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.

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[sbiscigl]

Looks like the build is failing, otherwise LGTM. If you have time address the comma-delim join re-usable method.

[sbiscigl]

oh you need to construct a comma delim list, theres a nice little trick for that using std::accumulate. we do it in other places when we need that.

std::accumulate(std::begin(m_clientConfig->sigV4aSigningRegionSet), 
  std::end(m_clientConfig->sigV4aSigningRegionSet), 
  Aws::String{},
  [](const Aws::String &acc, const Aws::String &item) -> Aws::String {
    const auto value = item;
    return acc.empty() ? value : acc + "," + value;
  })

Might even be worth it to create a StringUtils::Join(...) function that we can use in the future.