Allow specifying initial value for AnimGraph string parameter in editor

[yuriy0]

The editor AnimGraph component allows you to specify the default values for some, but not all, AnimGraph parameter types.

This change adds support for specifying the default value for string AnimGraph parameters in the Editor AnimGraph component.

Issue #, if available:

Description of changes:

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[AMZN-puvvadar]

Hey @yuriy0,

We've passed this on for internal review, thanks for the submission!