Fix smithy signer ctors for older gcc

[sbiscigl]

Description of changes:

On older GCC core builds with the error

In file included from aws-sdk-cpp/src/aws-cpp-sdk-core/include/smithy/identity/auth/built-in/SigV4AuthScheme.h:13:0,
                 from aws-sdk-cpp/tests/aws-cpp-sdk-core-tests/smithy/client/SmithyClientTest.cpp:8:
aws-sdk-cpp/src/aws-cpp-sdk-core/include/smithy/identity/signer/built-in/SigV4Signer.h: In constructor 'smithy::AwsSigV4Signer::AwsSigV4Signer(const SigV4AuthSchemeParameters&)':
aws-sdk-cpp/src/aws-cpp-sdk-core/include/smithy/identity/signer/built-in/SigV4Signer.h:24:113: error: could not convert 'parameters' from 'const SigV4AuthSchemeParameters {aka const smithy::DefaultAuthSchemeResolverParameters}' to 'Aws::String {aka std::basic_string<char>}'
             :  m_parameters{parameters},legacySigner{nullptr, parameters.serviceName.c_str(), *parameters.region}
                                                                                                                 ^
In file included from aws-sdk-cpp/src/aws-cpp-sdk-core/include/smithy/identity/auth/built-in/SigV4aAuthScheme.h:13:0,
                 from aws-sdk-cpp/tests/aws-cpp-sdk-core-tests/smithy/client/SmithyClientTest.cpp:12:
aws-sdk-cpp/src/aws-cpp-sdk-core/include/smithy/identity/signer/built-in/SigV4aSigner.h: In constructor 'smithy::AwsSigV4aSigner::AwsSigV4aSigner(const SigV4aAuthSchemeParameters&)':
aws-sdk-cpp/src/aws-cpp-sdk-core/include/smithy/identity/signer/built-in/SigV4aSigner.h:36:39: error: could not convert 'parameters' from 'const SigV4aAuthSchemeParameters {aka const smithy::DefaultAuthSchemeResolverParameters}' to 'Aws::String {aka std::basic_string<char>}'
             :  m_parameters{parameters}

Passing DefaultAuthSchemeResolverParameters to singers in the ctors was out of convenience beforehand, so this moves the ctors to use service name and region as that is what is required in the signers

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