This change adds an exporter for [InfluxData line protocol](https://docs.influxdata.com/influxdb/v2.0/reference/syntax/line-protocol/), the native wire transfer protocol for [Telegraf](https://www.influxdata.com/time-series-platform/telegraf/) and [InfluxDB](https://www.influxdata.com/products/influxdb/).

The conversion logic lives in [package `otel2influx`](https://github.com/influxdata/influxdb-observability/tree/main/otel2influx), and is also imported by the [proposed Telegraf OpenTelemetry input plugin](influxdata/telegraf#9077). The objective of this strategy is to create a canonical mapping, and an open-source implementation, of OpenTelemetry <-> Telegraf/InfluxDB translation.

I am not proposing that `otel2influx`, in its current state, be the canonical mapping. Let's discuss that topic separate from this exporter.

One shortcoming of this approach is that both `otel2influx` and `opentelemetry-collector` package their own generated protocol buffer objects. This means that every OTLP protobuf object is serialized and deserialized in this exporter. I propose that `opentelemetry-collector` instead depend on `opentelemetry-proto-go`, for the benefit of this exporter and other third-party packages.
```golang
// r is a protobuf from otel2influx
// td is a protobuf wrapper from otelcol
var r otlpcollectortrace.ExportTraceServiceRequest
if protoBytes, err := td.ToOtlpProtoBytes(); err != nil {
	return err
} else if err = proto.Unmarshal(protoBytes, &r); err != nil {
	return err
}
```

**Link to tracking Issue:** #2951

**Testing:** Little testing so far. The translation logic should be tested in [package `otel2influx`](https://github.com/influxdata/influxdb-observability/tree/main/otel2influx) (but is not currently).

**Documentation:** README.md included in new exporter.