[JAVA][Webclient] Add constructor to allow custom WebClient to be passed into ApiClient.

Created by: hfgbarrigas

Description

Add a new constructor in the JAVA Webclient mustache template allowing a custom webclient to be used instead of the default one. This allows for flexible customisation of the client, e.g, timeouts.

openapi-generator version

maven-plugin -> 3.2.1

Suggest a fix/enhancement

I have a simple branch and willing to merge.