Add VL_INSTANCE_TLS_SKIP_VERIFY option to skip TLS certificate verification

[mkobozev]

What does this PR do?

Adds a new environment variable VL_INSTANCE_TLS_SKIP_VERIFY that allows
skipping TLS certificate verification when connecting to VictoriaLogs instances over HTTPS.

Why is this needed?

Many internal/on-premise deployments use self-signed or corporate CA certificates
that are not trusted by the system certificate store. Without this option, the MCP
server cannot connect to such VictoriaLogs instances over HTTPS.

Changes

• cmd/mcp-victorialogs/config/config.go: new tlsSkipVerify field, env var parsing, getter
• cmd/mcp-victorialogs/main.go: configure http.DefaultTransport with InsecureSkipVerify
  when enabled; uses .Clone() to preserve default transport settings (connection pooling, timeouts, keep-alives)
• README.md: added variable to configuration table with a security warning

Notes

• Disabled by default — no behavioral change for existing users
• Logs a warning via slog.Warn when enabled
• Follows existing naming convention (VL_INSTANCE_*)