Fix assignment operator in Optional

[sfod]

Issue #, if available:

There are multiple issues with Optional's assignment operators and related tests:

• template <typename U = T> Optional &operator=(U &&u) is too greedy, so it's used whenever argument type differs from other assignment operators types:

  Optional<Foo> o1;
  Optional<Foo> o2;
  o2 = o1;  // resolves to the "greedy" assignment operator, because o1 is not const ref
  

• template <typename U = T> Optional<T> &operator=(const Optional<U> &other) and template <typename U = T> Optional<T> &operator=(Optional<U> &&other) use other's private members

  Specializations of a template class are completely unrelated, so they can't access each other's private members.

• Tests for assignment operators for Optional class check constructors instead.

  Tests use the following expressions to check assignment operators:

  Optional<Foo> o = another_optional;
  

  But constructor is used here.

The issues can be found in the first commit's CI in this PR. For example, the following code from the tests

Aws::Crt::Optional<CopyMoveTester> copyAssignedOptional;
Aws::Crt::Optional<CopyMoveTester> tester = CopyMoveTester();
copyAssignedOptional = tester;

produces these errors:

/root/aws-crt-cpp/include/aws/crt/Optional.h: In instantiation of Aws::Crt::Optional<T>& Aws::Crt::Optional<T>::operator=(U&&) [with U = Aws::Crt::Optional<CopyMoveTester>&; T = CopyMoveTester]:
/root/aws-crt-cpp/tests/OptionalMemorySafetyTest.cpp:219:36:   required from here
/root/aws-crt-cpp/include/aws/crt/Optional.h:45:30: error: no match for operator= (operand types are CopyMoveTester and Aws::Crt::Optional<CopyMoveTester>)
                     *m_value = std::forward<U>(u);

Description of changes:

• Make greedy assignment operator less greedy by enabling it only if its parameter type is not Optional.
• Fix member access in other assignment operators.
• Fix tests.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.