Consolidating proto/ and common functions into opentelemetry-common/

Propose to move proto/ and common functions into opentelemetry-common/ so that otlp, otlp-metrics, and otlp-logs exporters
can use the centralized utilities.

1. Current opentelemetry-proto is v1.10, while our proto is still v1.8.0. Centralizing proto to common avoids managing three separate proto/ folders.
2. There are common utilities such as prepare_endpoint, prepare_headers, etc. Consolidating them together prevents issues like https://github.com/open-telemetry/opentelemetry-ruby/pull/2158.

Other things to consider:
otlp and otlp-http are confusing—people may check otlp-http rather than otlp, and otlp-http still needs to be updated if other otlp exporters change.

1. Rename otlp → otlp-trace (similar to otlp-metrics and otlp-logs); if gRPC support is added in the future, consider adding the gRPC exporter inside otlp-trace/.
2. Add a notice that otlp-http is not maintained and redirect users to otlp.

Start by mapping the current proto/ and common utilities such as prepare_endpoint and prepare_headers across opentelemetry-proto, otlp, otlp-metrics, and otlp-logs. Review how otlp and otlp-http are organized and maintained. Done means the consolidation, naming, and maintenance notice decisions are agreed and implemented without duplicated proto or common-function locations.