Prevent mutating the client base URL

[SteppingHat]

In instances where a single use of the client is maintained throughout the lifetime of an application, there are operations that mutate the state of the client making it unusable for future requests.

For example, when sending three SMS messages a minute apart from each other, the log output shows the following:

{"level":"debug","REST CLIENT":"Do","time":"2024-06-20T18:37:00+10:00","message":"Sending POST request to https://api.smsglobal.com/v2/sms"}
...
{"level":"debug","REST CLIENT":"Do","time":"2024-06-20T18:38:00+10:00","message":"Sending POST request to https://api.smsglobal.com/v2/sms/sms"}
...
{"level":"debug","REST CLIENT":"Do","time":"2024-06-20T18:39:00+10:00","message":"Sending POST request to https://api.smsglobal.com/v2/sms/sms/sms"}

The issue is that when appending the path to the baseURL, it mutates the baseURL.

smsglobal-go/internal/pkg/client/client.go

Lines 68 to 69 in 4629421

	// append path to existing path "/v2"
	c.BaseURL.Path = p.Join(c.BaseURL.Path, rel.Path)

This PR changes the way URL's are generated so that they are no longer mutating the client's BaseURL object.
Instead, it now creates it's own object used to create a request.

[SteppingHat]

Bump