[Java] Add add constructor to BYO OkHttpClient

[jonnii]

The intention of this change is to allow someone to supply their own OkHttpClient to the ApiClient in the event that they don't want to pay the price for creating a new client even when if they're going to setHttpClient.

A specific example of this is if you have multiple ApiClients (e.g. imagine a user service and an avatar service) which have different API contracts but could share the same DNS/caching/resiliency settings etc...

I appreciate that this could be solved with a custom template but this felt useful enough to contribute.

PR checklist

• Read the contribution guidelines.
• If contributing template-only or documentation-only changes which will change sample output, build the project before.
• Run the shell script(s) under ./bin/ (or Windows batch scripts under.\bin\windows) to update Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit, and these must match the expectations made by your contribution. You only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the code or mustache templates for a language ({LANG}) (e.g. php, ruby, python, etc).
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[jonnii]

@wing328 Any suggestions on what else is required to get this in and/or feedback would be greatly appreciated!