[FEATURE] Provides a method to replace baseURL for external unit testing

[MangataL]

Is your feature request related to a problem? Please describe.
other packages using the client cannot write a unit test because the baseURL variable cannot be modified and can only access the real tmdb address.

Describe the solution you'd like
add a method to modify baseURL

Describe alternatives you've considered

Additional context

[cyruzin]

@MangataL https://github.com/cyruzin/golang-tmdb/releases/tag/v1.6.4

I added two new functions:

SetCustomBaseURL(url string)
GetBaseURL() string