Support for `OTEL_EXPORTER_ZIPKIN_TIMEOUT`

[Kielek]

Feature Request

Support for OTEL_EXPORTER_ZIPKIN_TIMEOUT based on https://github.com/open-telemetry/opentelemetry-specification/blob/dd47c009da1de335c540decc5d5930e46ea61916/specification/sdk-environment-variables.md?plain=1#L246

Additional Context

Found while working on https://github.com/open-telemetry/opentelemetry-dotnet-instrumentation/issues/2009

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[TimothyMothra]

I spoke to @Kielek offline, this issue is tracking an otel spec. I'm moving this to the Future backlog.