Swap dynamo client to use a smithy base class

[sbiscigl]

Description of changes:

Per the work to move to a more common architecture between different AWS SDKs. This moves the Dynamo DBClient to use the smithy identity based flow instead of the legacy workflow,

Examples other SDKs updating their flow:

• Java V2 change
• JS V3 example

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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.
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[DmitriyMusatkin]

nit: do we do it in other places? should we hide this behind a helper returning a ref to this?

[DmitriyMusatkin]

i see its used below?
something like constexpr char str[] = "foo"; wouldnt work here?

[sbiscigl]

good call

[sbiscigl]

this actually leads to gcc errors about odr

aws-sdk-cpp/generated/src/aws-cpp-sdk-dynamodb/include/aws/dynamodb/DynamoDBClient.h:39:26: error: 'Aws::DynamoDB::DynamoDBClient' has a field 'Aws::DynamoDB::DynamoDBClient::' whose type uses the anonymous namespace [-Werror=subobject-linkage]
--
1223 | class AWS_DYNAMODB_API DynamoDBClient : smithy::client::AwsSmithyClientT<Aws::DynamoDB::SERVICE_NAME,
1224 | ^~~~~~~~~~~~~~
1225 | aws-sdk-cpp/generated/src/aws-cpp-sdk-dynamodb/include/aws/dynamodb/DynamoDBClient.h:39:26: error: 'Aws::DynamoDB::DynamoDBClient' has a base 'smithy::client::AwsSmithyClientT<(& Aws::DynamoDB::SERVICE_NAME), Aws::DynamoDB::DynamoDBClientConfiguration, smithy::SigV4AuthSchemeResolver<>, Aws::Crt::Variantsmithy::SigV4AuthScheme, Aws::Endpoint::EndpointProviderBase<Aws::DynamoDB::DynamoDBClientConfiguration, Aws::DynamoDB::Endpoint::DynamoDBBuiltInParameters, Aws::Endpoint::ClientContextParameters>, smithy::client::JsonOutcomeSerializer, Aws::Utils::Outcome<Aws::AmazonWebServiceResultAws::Utils::Json::JsonValue, Aws::Client::AWSErrorAws::Client::CoreErrors > >' whose type uses the anonymous namespace [-Werror=subobject-linkage]
1226 | cc1plus: all warnings being treated as errors
1227 | [618/2688] Building CXX object generated/src/a

we ran into other things like this, and just looking around, i think it boils down to "we cant have the deifnition in a header"