Add NewSecretKeyFromEd25519Bytes

[oldgreen]

https://github.com/w3f/schnorrkel/blob/ab3e3d609cd8b9eefbe0333066f698c40fd09582/src/keys.rs#L495-L533

This is useful for working with the exported polkadot.js account JSON file.

[CLAassistant]

All committers have signed the CLA.

[jimjbrettj]

@oldgreen Thanks for this! All looks good to me, I believe you just still need to sign the contributor agreement