ref: improve telemetry processor encoding

[giortzisg]

Description

This PR improves the abstraction of adding and serializing telemetry items for the telemetry processor.

The changes include:

• refactoring the scheduler around ToEnvelope instead of ToEnvelopeItem. Creating the envelope should be the responsibility of the event itself. This changes was needed for cases like attachments, where events should handle any additional encoding that should happen on the envelope.
• abstract batchable logic to the ItemContainer. Added the abstraction layer of item container and removed metric_batch and log_batch which were sharing the same underlying logic.
• modified report.RecordItem to require one less typecast.

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