Fix encoding of sign() JS API function

[takuro-sato]

We are aiming to make the JS crypto API to follow the Web Crypto API in behavior.
To do so, #2735 changed ccf.crypto.verifySignature to accept IEEE P1363 as signature's encoding format from DER when ECDSA is used.
But recently added ccf.crypto.sign doesn't follow it and creates DER encoded signature and we need to fix it.
Also we haven't checked encoding to use for EdDSA, so we need to check it from Web Crypto API spec.

[achamayou]

@takuro-sato we need to check for RSA too.

[takuro-sato]

TODOs

• Sign with P1363 encoding for ECDSA
• Check if the encoding for RSA and EdDSA is compatible with Web Crypto API (and fix if not)