Tweak OpenTelemetry config to rely on default OTel SDK env vars

[kylewlacy]

This PR builds on #92 by tweaking how OpenTelemtry is configured. This removes $BRIOCHE_JAEGER_ENDPOINT completely, and instead adds a new $BRIOCHE_ENABLE_OTEL env var. When $BRIOCHE_ENABLE_OTEL=1, then it will use the default OpenTelemtry SDK config to pick the endpoint and URL, see https://opentelemetry.io/docs/languages/sdk-configuration/otlp-exporter/#otel_exporter_otlp_protocol

Previously, this command from #92 (comment):

BRIOCHE_JAEGER_ENDPOINT=http://jaeger:4318/v1/traces cargo run -- fmt -p /tmp/brioche-packages/packages/bat

...would now become this:

BRIOCHE_ENABLE_OTEL=1 OTEL_EXPORTER_OTLP_ENDPOINT=http://jaeger:4318 cargo run -- fmt -p /tmp/brioche-packages/packages/bat