[otlp-transformer] reduce public API surface

[pichlermarc]

Description

The @opentelemetry/otlp-transformer package contains utilities to serialize signals data to and from OTLP (binary protobuf) or OTLP (json). As all OTLP exporters depend on this package, stabilization of @opentelemetry/otlp-transformer is required for OTLP exporter stability.

The API surface currently consists of 40+ types and functions. Reducing the API surface is required to enable optimizations in the serialization process in terms memory use, performance, and bundle size without being constrained by a large stable public API.

Goal of this issue is to:

Move the full serialization process to @opentelemetry/otlp-transformer, this way we will be able to reduce the API surface to:

• an ISerializer interface
• one serializer implementation per signal (metrics, logs, traces) and type (binary protobuf, json)
• one response type per signal.