Add EIP-5791: Physical Backed Tokens

[2pmflow]

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[eth-bot]

A critical exception has occurred: Message: pr 5791 is already merged; quitting (cc @alita-moore, @mryalamanchi)

[ivanmmurciaua]

That's an awesome project and very interesting real value for decentralized world

[edddyguo]

Awesome project！

[2pmflow]

@Pandapip1 I moved a chunk to Motivation here https://github.com/ethereum/EIPs/pull/5791/commits/a14103cac4cf6232fce2647c72942f63e04ce460, but was having trouble finding other parts of the spec that are a better fit in Motivation than their current respective sections, based on the prompt for Motivation in the template (The motivation section should describe the "why" of this EIP.). To me, the "why" is to solve for the two enumerated problems, and the rest is "how" the spec solves those problems. Could you clarify what chunks you think are a better fit for the "why"? Happy to make changes as appropriate.

[YAYLABS-Snoo]

I think it will help develop various interesting contents that can be done offline as well as trading. 

[2pmflow]

@Pandapip1 thanks for detailed review. Feedback addressed in latest