feat: Add content-type sign request response

[congminh1254]

Closes: SDK-2253

[coveralls]

Pull Request Test Coverage Report for Build #1937

• 35 of 50 (70.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 69.853%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/com/box/sdk/BoxSignRequestSigner.java	35	50	70.0%

Totals
Change from base Build #1909:	0.02%
Covered Lines:	7271
Relevant Lines:	10409

---

💛 - Coveralls

[antusus]

Let's improve this a little

[antusus]

Maybe let's use switch which is not used in SDK and I have no idea why...
And we should provide details about the faulty value

static BoxSignRequestInputContentType fromJSONString(String jsonValue) {
    switch (jsonValue) {
        case "initial":
            return Initial;
        case "stamp":
            return Stamp;
        case "signature":
            return Signature;
        case "company":
            return Company;
        case "title":
            return Title;
        case "email":
            return Email;
        case "full_name":
            return FullName;
        case "first_name":
            return FirstName;
        case "last_name":
            return LastName;
        case "text":
            return Text;
        case "date":
            return Date;
        case "checkbox":
            return Checkbox;
        default:
            throw new IllegalArgumentException(
                format("The provided JSON value '%s' isn't a valid BoxSignRequestInputContentType.", jsonValue)
            );
    }
}

[antusus]

👍