Add `core/eosio` library method to Convert String to `public_key`

[bhazzard]

As a smart contract developer, I want a CDT core/eosio library method that converts a string (e.g., from a transfer memo) into a public_key object, so that I can parse and use public keys directly in contracts without relying on fragile workarounds or requiring a hard fork.

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Context

Currently, there's no straightforward way to convert a public key string (like "EOS6MRy...") into a usable public_key object within a smart contract. Developers often resort to manual base58 decoding and checksum verification, which is error-prone and not ideal for contract logic.

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Problem

Several smart contracts—such as openvaccount, openaccounts, makeaccounts, and signupeoseos—rely on parsing public keys from transfer memos to create new accounts. These contracts currently support only EOS-prefixed keys due to the absence of a reliable method to handle newer key formats (e.g., PUB_K1, PUB_R1). This limits both flexibility and future compatibility.

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Acceptance Criteria

Introduce a CDT core/eosio library method (e.g., string_to_public_key) that parses a public key string into a public_key object.

Ensure the method supports all valid EOS key formats, including EOS, PUB_K1, and PUB_R1.

Provide documentation with usage examples and integration guidance.

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Notes

By handling this at compile time via a CDT/eosio library method, we avoid the need for a hard fork while still enabling cleaner and safer contract development. This improves developer experience and allows support for a wider range of key formats in existing and future smart contracts.

[heifner]

Could this not just be a cdt core/eosio library method? Not sure a host function is needed.

[bhazzard]

@nsjames any thoughts on Kevin's suggestion above?

[nsjames]

Yes, it could. It would likely be more performant as a host function, but in the grand scheme of things probably negligible.

[yarkinwho]

Yes, as well as the signature I believe.
There's quite a few problems if one try to use a public_key or signature data structure directly as part of the action arguments. Using byte serialization will work but this is surely not as intuitive as the strings.