Changed url validation

[tobias-kuendig]

Because of the base API url validation, it is currently not possible to use a url like http://localhost/my-proxy/v3.

My specific use case is to proxy the requests to the Mailgun api via a https://my-internal-server/mailgun/v3 base. I cannot access the Mailgun API directly in the network but I have access to my-internal-server.

Using https://my-internal-server/mailgun/v3, I get an error that reads BaseAPI must end with a /v2, /v3 or /v4, which it does in my example, but the regex to validate the url is ^/v[2-4].*, which checks if it starts with the version segment.

Alltough I think this check could be removed completely (a user will notice the wrong URL when the request is sent, like they would notice a wrong username/password) this PR proposes to change the regex to just /v[2-4].* so only the presence of a /v3 segment is validated, no matter where in the URL.

I guess it would be better to validate the URL somewhere where the actual apiBase field is available so we could actually validate if it ends in /v3 but I did not find a way to implement it without breaking the api. The httpRequest struct does not know about the apiBase but only sees the fully constructed url which makes the validation for a specific format harder.

[thrawn01]

thank you for making this change and explaining your use case!