[Geneva Exporter] Restructure code to allow non-otlp payload encoding

[lalitb]

We are forcing everyone to transform the data into the OTLP structs. This is inefficient if our user does not deal with OTLP data in memory. We could update the GenevaClient::upload_logs method to operate on a trait instead. We could then have different implementations for this trait: for OTLP structs, for SDK's LogBatch etc.

Just something to keep in mind. Not necessarily required for this PR.

Originally posted by @utpilla in #261 (comment)