feat(curl): generate curl cmd for request && example for curl cmd

[ahuigo]

I've extracted curl code from my previous PR(#734).
Could you review this code?

New feature: generate curl command

Example: https://github.com/ahuigo/resty/blob/curl/v2/examples/debug_curl_test.go

var curlCmdExecuted string
client := resty.New()
_, err := client.R().
    SetResultCurlCmd(&curlCmdExecuted).
    Get("https://httpbin.org/get")

// Explore curl command
fmt.Println("Curl Command:", curlCmdExecuted)

Output

$ go test -run '^TestDebugCurl$' github.com/go-resty/resty/v2/examples -v
=== RUN   TestDebugCurl
    debug_curl_test.go:29: curlCmdUnexecuted:  
    curl -X GET -H 'Content-Type: application/json' -H 'Cookie: count=1' -H 'User-Agent: go-resty/2.12.0 (https://github.com/go-resty/resty)' -d '{"name":"Alex"}' 'http://unexecuted-url'

    debug_curl_test.go:41: curlCmdExecuted:  
    curl -X POST -H 'Accept: application/json' -H 'Content-Type: application/json' -H 'Cookie: count=1; count=1' -H 'User-Agent: go-resty/2.12.0 (https://github.com/go-resty/resty)' -d '{"name":"Alex"}' http://127.0.0.1:56426/post
--- PASS: TestDebugCurl (0.00s)