Allow prefer config

[wenovus]

• Essentially just added a IsShadowPath() function alongside IsConfig() to allow generation/unmarshal to happen correctly.
• Added simple bash script to copy ygnmi_test.go to ygnmi_preferconfig_test.go for testing.

This allows existing ygot code that prefers config paths to transition to using ygnmi.

[coveralls]

Pull Request Test Coverage Report for Build 6805509420

• 92 of 101 (91.09%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 82.305%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pathgen/pathgen.go	29	32	90.63%
pathgen/gnmigen.go	24	30	80.0%
<!--	Total:	92	101	91.09%	-->

Totals
Change from base Build 6805466690:	0.05%
Covered Lines:	2000
Relevant Lines:	2430

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💛 - Coveralls