Replace seqcli with Rust-based OTLP emitter

[KodrAus]

This project currently forwards decoded GELF events to Seq via seqcli ingest. This was a good way to bootstrap the project, but the .NET dependency introduces unstable runtime behavior. It's also a big additional moving part to manage.

Now that Seq supports native OTLP ingestion, we can remove the seqcli dependency and replace it with a Rust-based OTLP emitter. The quickest way to bootstrap this would probably be via tonic, but we could also roll our own gRPC if we need to for custom header support.

[nblumhardt]

Or emit + emit_otlp?