[Feature Request] Refactor and limit all v5 config structure definitions

[hellodword]

Related issue: #2900 (comment)

We have three strict¹ styles of keys in v5:

packetEncoding and fallback_tag, also enums as UseIP

They're really confusing and really effect the user experience, a lot.

And I believe the chaos will make a mess in the future even for developers, so this is a proposal.

BTW, I do not agree with this kind of strict, while @xiaokangwang said less forgiving to reduce complexity²

Any idea :)

Footnotes

1. https://github.com/v2fly/v2ray-core/pull/2897 ↩

2. https://github.com/v2fly/v2ray-core/commit/b583ef45b1beb57d4321c2e8a06c0bbcf28a1c26 ↩