opentelemetry-exporter-otlp-pyproto-http: drop requests, use urllib

[ocelotl]

Summary

• Replace requests.Session-based HTTP transport in opentelemetry-exporter-otlp-pyproto-http with stdlib urllib.request/urllib.error, so the package no longer depends on requests.
• Shared _build_ssl_context/_post helpers added to _common.py, used identically by trace_exporter, _log_exporter, and metric_exporter.
• The opentelemetry_otlp_credential_provider entry-point mechanism's isinstance(maybe_session, requests.Session) type check is commented out (not deleted) since there's no requests.Session type left to validate against.
• The session= constructor parameter and its wiring are removed from all three exporters, since there's no more Session object for it to populate. _load_session_from_envvar itself remains defined in _common.py but is now unreferenced by the exporters — this is a real, partial feature regression: the credential-provider plugin mechanism (e.g. a GCP-auth-style pluggable session) is no longer wired into the request path.

Test plan

• uv run pytest tests/ — 91 passed
• Confirmed no requests import remains in src/, and requests is not loaded as a module at runtime
• uv.lock regenerated, no longer lists requests as a dependency of this package