Enable setting OkHttpClient for FirebaseFunctions

[svenjacobs]

This PR fixes #5404. I deliberately did not add the OkHttpClient instance to the getInstance factory methods because this would mean four additional variants of getInstance. I believe setting the HTTP client instance is a rare use case, so this is solved through an additional setter.

[svenjacobs]

I forgot to add nullable annotations. I will add them later. Done.

[svenjacobs]

The PR has been rebased onto the Kotlin migration of main.

[svenjacobs]

@thatfiredev Some explanation:

I created the setOkHttpClient and getOkHttpClient functions in Kotlin although I didn't have to. I could have just opened the variable client. However from the naming it might not be obvious what client is. There are three possibilities now.

1. Leave the setter and getter functions as is
2. Remove the functions and make client public accessible

public var client: OkHttpClient

Additionally we could add more descriptive names via annotations here. But these would only be visible from Java.

public var client: OkHttpClient = OkHttpClient()
  @JvmName("setOkHttpClient")
  set
  @JvmName("getOkHttpClient")
  get

3. Remove the functions, rename client to okHttpClient and make it public accessible. We would have a descriptive name for both Kotlin and Java.

What do you think?