Add Otel/STEF project

Otel/STEF (Sequential Tabular Encoding Format) is a new data format and network
protocol for OpenTelemetry data.

For the target use-cases Otel/STEF outperforms both OTLP and Otel Arrow: Otel/STEF is smaller and/or faster.

See stef.md for details: benchmarks and comparisons to other formats,
links to prototypes and description of project goals.