feat: add SNMP trap provider

[Treasure520520]

/claim #2112

Fixes #2112

Summary

Adds a native SNMP provider that can receive SNMP v1/v2c traps over UDP and convert them into Keep alerts.

The implementation is intentionally dependency-light: it uses a small BER decoder plus a UDP listener, so Keep can receive common traps without adding a third-party SNMP runtime dependency.

What changed

• Added SnmpProvider with configurable listen_host, listen_port, community, queue size, and socket timeout.
• Supports SNMP v1 trap PDUs and SNMP v2c trap/inform PDUs.
• Maps well-known traps such as linkDown, linkUp, coldStart, warmStart, and authenticationFailure into Keep status/severity.
• Decouples UDP packet reception from Keep API pushes using a bounded queue and worker thread.
• Filters incoming traps by configured community string.
• Adds webhook-style formatting support for SNMP trap forwarders.
• Adds tests for v1 decoding, v2c decoding, webhook formatting, and queued alert pushing.
• Adds provider docs and mock alerts.

Demo

Short demo asset: https://github.com/Treasure520520/keep/releases/download/keep-snmp-provider-demo-2112/keep-snmp-provider-demo.gif

Validation

poetry run black --check keep/providers/snmp_provider tests/test_snmp_provider.py
poetry run ruff check keep/providers/snmp_provider tests/test_snmp_provider.py
poetry run python -m pytest tests/test_snmp_provider.py -q
git diff --check

The focused test run passes with 4 SNMP provider tests.

Notes

• Default trap port is 162, but the docs recommend using an unprivileged port such as 1162 for local development.
• SNMPv3 auth/privacy is not included in this PR.

[CLAassistant]

All committers have signed the CLA.

[Treasure520520]

CLA is now signed and the issue-link/title/security checks are green. This PR is ready for maintainer review from my side. Thanks!