allow overriding custom http client properties

[jrhizor]

I would like to be able to customize client-level settings for the underlying OkHttpClient while still using the majority of defaults in HttpClientUtils.createHttpClient.

The specific use case I'm interested in is customizing the connection pool used by the client because we're running into an issue where it blocks program closure by taking 5 min to give up an idle connection. If this was modifiable we could customize this:

httpClientBuilder.connectionPool(new ConnectionPool(5, 30, TimeUnit.SECONDS));

The reason I'd like to be able to modify this is because HttpClientUtils.createHttpClient sets a lot of useful defaults. I don't want to have to create and maintain a duplicate of HttpClientUtils.createHttpClient in my code base just so I can modify this.

This can be achieved by making HttpClientUtils.createHttpClient public. Additional overrides can be supplied via the additionalConfig consumer.

[manusa]

Should have been fix by #3171