Implement a simplified version of STCP functionality.
* **Client Proxy Configuration and Handling**
- Add `STCPProxyConfig` to the `pxyConfs` slice in `client/proxy/general_tcp.go`.
* **Client Visitor Configuration and Handling**
- Add `STCPVisitorConfig` to the `cfg` struct in `client/visitor/stcp.go`.
* **Command Registration and Initialization**
- Add `STCP` to the `proxyTypes` and `visitorTypes` slices in `cmd/frpc/sub/proxy.go`.
* **Configuration Examples**
- Add example configurations for `STCP` proxy and visitor in `conf/frpc_full_example.toml`.
- Add example configurations for `STCP` proxy and visitor in `conf/legacy/frpc_legacy_full.ini`.
* **Configuration Structures**
- Add `STCPProxyConfig` struct and include it in the `proxyConfigTypeMap` in `pkg/config/v1/proxy.go`.
- Add `STCPVisitorConfig` struct and include it in the `visitorConfigTypeMap` in `pkg/config/v1/visitor.go`.
* **Configuration Validation**
- Add validation for `STCPProxyConfig` in `pkg/config/v1/validation/proxy.go`.
- Add validation for `STCPVisitorConfig` in `pkg/config/v1/validation/visitor.go`.
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