List selectable signature methods
Returns the signature methods you can offer a signer, with display labels — the generic methods plus every currently-available eID scheme.
This is the pickable set, not every accepted value. NONE and MANUAL are still accepted on requiredSignature but are omitted here because the app derives them rather than offering them: NONE is what a party gets when its role isn’t a signing role, and MANUAL is for an imported archive document that was already signed on paper.
Build signature-method pickers from this rather than hard-coding the list: eID schemes are switched on per market over time, and sending a scheme that isn’t available yet is rejected with “This eID scheme is not yet available”.
BANKID is Swedish BankID (internally SE_BANKID); the legacy value is what the API emits and is accepted permanently.

