Use `WithSpanName()` as parent context

[scriptnull]

Right now, we have <Consumer/Producer/Transformer>.name (<Worker>.name) as the parent span and WithSpanName() as the child span. The parent span is having other spans too (like error, wait etc. - refer code for exact info)

• WithSpanName is optional right now. We can keep it optional.
• The parent span will be <Worker>.name by default.
• If WithSpanName is called, then we can use that as the parent span.
• The internal spans will have a standard suffix like <worker>.name.work or <worker>.name.error.
• If needed, we can implement WithSpanSeparator(sep string) that can be used to configure the separator to customize <worker>.name.work to be <worker>.name <some separator> work