feat: export (const.)core.v2rayKey

[rurirei]

as for #698

ctx initialization for core.Dial() requires this, as from core.RequireFeatures() introduced in dispatcher.newFakeDNSSniffer().

ctx = context.WithValue(ctx, vcore.V2rayKeyValue, p.v)

[kslr]

Thanks for your Work. :)

[xiaokangwang]

@rurirei

I am really sorry that I have to revert your change.

This variable cannot be make a public API of V2Ray, it may subject to change in the future. Instead, I have updated the code so that Dial will automatically set instance context variable. (fc73774)

Will this work in your case?

[rurirei]

@rurirei thanks, i'll try it.