[ProtocolTests] #2 Serialize timestamps and enums nested into lists or maps of Query String

[SergeyRyabinin]

Issue #, if available:
Protocol tests implementation highlighted an issue in serialization of Query Strings
when the query string has a list of Timestamps.

Description of changes:
Changes how Vector serialization to a query string is done
from

    if(m_queryTimestampListHasBeenSet)
    {
      for(const auto& item : m_queryTimestampList)
      {
        ss << item; // LOOK HERE
        uri.AddQueryStringParameter("TimestampList", ss.str());
        ss.str("");
      }
    }

to

    if(m_queryTimestampListHasBeenSet)
    {
      for(const auto& item : m_queryTimestampList)
      {
        ss << item.ToGmtString(Aws::Utils::DateFormat::ISO_8601); // LOOK HERE
        uri.AddQueryStringParameter("TimestampList", ss.str());
        ss.str("");
      }
    }

No actual diff is generated on existing prod service models.
This will fix only the protocol test client build and will be tested by proto tests once merged.

Check all that applies:

• 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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