Support `http/protobuf` transport for ingesting OpenTelemetry logs

## What?

The OpenTelemetry input should support `http/protobuf` as a transport for OTLP log messages. It currently only supports `grpc`. Support should be implemented as either an additional option for the existing input type, or a new input type, supporting the `http/protobuf` transport should be added.

## Why?

The OpenTelemetry input only supports the `grpc` OTLP transport. The [exporter specification](https://github.com/open-telemetry/opentelemetry-specification/blob/v1.46.0/specification/protocol/exporter.md#specify-protocol) states the following:

> SDKs SHOULD support both grpc and http/protobuf transports and MUST support at least one of them. If they support only one, it SHOULD be http/protobuf.

The OpenTelemetry Java Instrumentation project, for instance, changed their default transport from `grpc` to `http/protobuf` with version [2.0.0](https://github.com/open-telemetry/opentelemetry-java-instrumentation/releases/tag/v2.0.0).

While `grpc` is still a good choice, it would be good to support the `http/protobuf` default transport, so that Graylog will work out of the box as an OpenTelemetry log sink for a wide variety of SDKs.

## Your Environment

* Graylog Version: 6.3.x