Add HTTP probing support

[anton-kuklin]

This PR adds support for HTTP probing. It provides an API based on httptrace callbacks.

Current list of configurable options:

• URL
• Host
• Method
• Headers
• Body
• Timeout
• CallFrequency (aka target RPS)
• MaxConcurrentCalls (aka number of call workers)

Available callbacks:

• OnReq
• OnResp
• OnDNSStart
• OnDNSDone
• OnConnStart
• OnConnDone
• OnTLSStart
• OnTLSDone
• OnWroteHeaders
• OnFirstByReceived

Misc config:

• Custom http client
• Logger

Basic usage example

httpCaller := probing.NewHttpCaller("https://www.google.com",
    probing.WithHTTPCallerCallFrequency(time.Second),
    probing.WithHTTPCallerOnResp(func(suite *probing.TraceSuite, info *probing.HTTPCallInfo) {
        fmt.Printf("got resp, status code: %d, latency: %s\n",
            info.StatusCode,
            suite.GetGeneralEnd().Sub(suite.GetGeneralStart()),
        )
    }),
)

// Listen for Ctrl-C.
c := make(chan os.Signal, 1)
signal.Notify(c, os.Interrupt)
go func() {
    <-c
    httpCaller.Stop()
}()
httpCaller.Run()

Based on issue: #88

[anton-kuklin]

CC: @davinci26

[SuperQ]

Going to merge this so we can iterate on it.