Make Signer Trait more flexible on asn1 formating

[Boxiao Cao]

The js_signer mod formats signature as asn1 in the rust code, because the current TypeScript implementation does not do the asn1 formatting.

However, there is a posibility that some JavaScript signer already includes the asn1 formatting. We should provide the flexibility to disable the asn1 formatting in the Rust code.

[Boxiao Cao]

Instead of having the single JsSigner::new, we can make the change to have two public constructors

impl JsSigner {
    // The JS function should be of type
    // `(input: Uint8Array) => Promise<Uint8Array>`.
    // The JS function returns the raw signature, which contains exactly 64 bytes.
    pub fn new(js_function: JsFunction) -> Self {
          todo!()
    }
    // The JS function should be of type
    // `(input: Uint8Array) => Promise<Uint8Array>`.
    // The JS function returns the signature in ASN1 format.
    pub fn new_with_asn1(js_function: JsFunction) -> Self {
          todo!()
    }
}

[Oliver Yu]

This sounds reasonable.