Add alpha channel attribute to color property type

[yuriy0]

Reflected color properties in the editor currently don't allow you to pick the alpha channel.

This change adds an attribute which can be applied to AZ::Color DataElements in the edit context which enables the alpha channel as follows:

...
                   ->DataElement(0, &MyClass::m_colour, "Some Color Value", "")
                   ->Attribute("AlphaChannel", true)
...

Issue #, if available:

Description of changes:

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[AMZN-puvvadar]

Hi @yuriy0 ,

Thanks for the submission. We'll take a look at this enhancement internally.