[WIP] Revert batch span processor environment variables changes

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Batch span processor env variables breaks C++20 aggregate initialization</issue_title>
<issue_description>@marcalff @nikhilbhatia08, this PR breaks a bunch of existing code without providing a sensible alternative.

Prior to that we were using the following syntax using C++20 aggregate initialization:

 auto processor = otel::BatchSpanProcessorFactory::Create(
            std::move(exporter),
            otel::BatchSpanProcessorOptions{
                .max_queue_size = config.batch().max_queue_size(),
                .schedule_delay_millis = std::chrono::milliseconds(config.batch().schedule_delay().MilliSeconds()),
                .max_export_batch_size = config.batch().max_export_batch_size(),
            }
        );

It does not seem that there is a sensible rewrite of this code.
Maybe this PR can be reverted?

Originally posted by @georgthegreat in #3661 (comment)

Proposal - I believe the issue comes from adding a user-defined constructor, which means BatchSpanProcessorOptions is no longer an aggregate type. That disables C++20 designated initializers, so patterns like .max_queue_size = ... stop working even though they worked earlier.
We can fix this without reverting anything by removing the constructor and using default member initializers along with a small set of static helper functions for the environment variable logic. This restores aggregate initialization while keeping the original behavior intact.
</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Batch span processor env variables breaks C++20 aggregate initialization #3778

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