new: Add Client.UseURL(...) method

[lgarber-akamai]

📝 Description

This PR adds a new client.UseURL(...) method that allows users to configure the scheme, host, and version of a client using a single URL.

For example:

client.UseURL("https://api.test1.linode.com/v4beta")

This is implemented in linodego as a convenience feature for projects that previously implemented this functionality manually (e.g. CCM).

✔️ How to Test

The following test steps assume you have pulled down this PR locally.

Unit Testing

make testunit

Manual Testing

1. In a linodego sandbox environment (e.g. dx-devenv), run the following:

package main

import (
    "context"
    "github.com/linode/linodego"
    "log"
    "os"
)

func main() {
    client := linodego.NewClient(nil)
    client.SetToken(os.Getenv("LINODE_TOKEN"))

    // This is intentionally set to an invalid URL
    if _, err := client.UseURL("https://api.test1.linode.com/v4beta"); err != nil {
        log.Fatal(err)
    }

    if _, err := client.ListInstances(context.Background(), nil); err != nil {
        log.Fatal(err)
    }
}

2. Ensure the run fails with an error similar to the following:

2024/03/28 13:17:38.300111 WARN RESTY Get "https://api.test1.linode.com/v4beta/linode/instances": dial tcp: lookup api.test1.linode.com: no such host, Attempt 1
2024/03/28 13:17:38.300217 ERROR RESTY Get "https://api.test1.linode.com/v4beta/linode/instances": dial tcp: lookup api.test1.linode.com: no such host
2024/03/28 13:17:38 [002] Get "https://api.test1.linode.com/v4beta/linode/instances": dial tcp: lookup api.test1.linode.com: no such host