Add support for v5 signatures

[jaherne-duo]

Description

This PR adds a new function JSONSignedCall(). Calls made through this function will use a v5 signature automatically.

Motivation and Context

V5 signatures and params encoded as JSON are required for the new policy API endpoints (see https://duo.com/docs/adminapi#policies). However, updating the existing SignedCall function would break existing golang client users. By adding a separate call, we can add support for future policy calls without disrupting existing code.

How Has This Been Tested?

Added new tests for the internal functions. Also verified the new call works with GETs and POSTs, both with and without params.

Example of how to use the new function:

func EnrollStatusV5(api *duoapi.DuoApi, userid string,
	activationCode string) (*authapi.EnrollStatusResult, error) {
	params := duoapi.JSONParams{
		"user_id":         userid,
		"activation_code": activationCode,
	}
	_, body, err := api.JSONSignedCall("POST", "/auth/v2/enroll_status", params)
	ret := &authapi.EnrollStatusResult{}
	if err = json.Unmarshal(body, ret); err != nil {
		return nil, err
	}
	return ret, nil
}

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[yizshi]

the canonicalization part looks right to me