Verify TLS by default with an opt-out

[eliteprox]

Summary

• The SDK verifies HTTPS certificates against the system trust store by default, across the shared request helpers (signer, discovery, runners, live payments), the lv2v /payment call, and trickle publish/subscribe.
• Set LIVEPEER_GATEWAY_VERIFY_TLS=0 (0/false/no/off), or livepeer_gateway.http.VERIFY_TLS = False, to turn verification off for the process when talking to a stock orchestrator that serves a self-signed certificate. The setting is read at call time.
• The opt-out context is built explicitly (create_default_context() + check_hostname=False + CERT_NONE), matching orch_info.py, instead of the private ssl._create_unverified_context().

Scope

• The setting is process-wide and all-or-nothing: there is no per-endpoint or per-call override.
• BYOC is unchanged. #69 removes that client.

Behavior change

Previously the SDK never verified HTTPS certificates. A deployment pointing at a stock self-signed orchestrator must set LIVEPEER_GATEWAY_VERIFY_TLS=0. The gRPC client remains unverified.

Supersedes #72, which was opened from feat/opt-in-tls-verification.

Test plan

• uv run --frozen --group test pytest tests/test_tls.py — 18 passed
• ruff check on http.py and tests/test_tls.py
• Snyk code scan on http.py — no issues
• Default (verification on): a call to a stock self-signed orchestrator fails certificate verification; a publicly trusted endpoint succeeds
• With LIVEPEER_GATEWAY_VERIFY_TLS=0: the self-signed orchestrator call succeeds