Add missing definition for GenerateSignature

[sbiscigl]

Issue #, if available:

issues/2599

Description of changes:

There was a missing definition for the public function

Aws::String GenerateSignature(const Aws::Auth::AWSCredentials& credentials,
  const Aws::String& stringToSign, 
  const Aws::String& simpleDate) const;

In the sigv4 signer. This PR adds the implementation and calls the corresponding private method.

We also tidy up and remove another un-implemented private fucntion.

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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