Reuse checksum on streaming retries + refactor ChecksumInterceptor

[sbaluja]

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Description of changes:

• On streaming operations, if the request payload was fully consumed and request checksum calculation was complete we will save the request checksum algorithm and value in memory to be reused for retry.
• Refactored the ChecksumInterceptor logic for setting checksum headers/values to be split across separate functions to follow single responsibility and allow for easier addition of checksum algorithms in the future.

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[sbiscigl]

this will initialize to
("", nullptr),
which is why you do
if (request.m_retryContext.m_requestHash.second == nullptr) {
later on to check for "absence".
would be better to have this as
std::shared_ptr<std::pair<Aws::String, std::shared_ptr<Aws::Utils::Crypto::Hash>>> m_requestHash
or
Aws::Crt::Optional<std::pair<Aws::String, std::shared_ptr<Aws::Utils::Crypto::Hash>>> m_requestHash

to better call out "absent" or "present"

[sbaluja]

Fair enough, was keeping it consistent with how we check for absence on the httpRequest itself - httpRequest->GetRequestHash().second == nullptr, but this sounds like a good change, will implement