[modem]: Add support for DTE-Tx hooks

[david-cermak]

Add before/after transmit hooks to DTE so users can toggle a GPIO (DTR / sleep pin) around every UART write; including internal PPP frames, LCP echo-replies, and other traffic that applications cannot wrap today. This enables UART power-save modes (Quectel QSCLK, u-blox UPSV, SIMCom CSCLK, etc.) without silent data loss.

• C++ API: DTE::set_transmit_hooks(before_tx, after_tx) hooks fire in all four DTE write paths (write(uint8_t*,size_t), write(DTE_Command), send(...), and inside command(...))

• *C API : esp_modem_set_transmit_hooks(dce, before_tx, after_tx, user_ctx) wraps the C++ hooks with an opaque context pointer

• Public terminal factory: create_uart_terminal() moved from private to public header, enabling C++ users to build custom DTE subclasses without private-include workarounds

• Fully additive, no breaking changes. When no hooks are registered, overhead is a single null-check per write.

Notes

• In CMUX mode, hooks may be called concurrently from PPP and AT-command threads. Users needing DTR gating under CMUX should use an atomic refcount.
• The hooks wrap only the actual terminal->write() call, not the full command-wait cycle, so DTR stays asserted only while bytes are in flight.

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Note

Medium Risk
Touches core DTE transmit paths and adds callback execution around every write, which could impact timing/concurrency (especially under CMUX) if hooks misbehave, though behavior is additive when hooks are unset.

Overview
Adds before/after transmit hooks to DTE (set_transmit_hooks) and wraps all DTE transmit paths (command(), write(), write(DTE_Command), send()) so callers can toggle GPIOs around every terminal write, including PPP/data traffic.

Exposes create_uart_terminal() as a public C++ API for building custom DTE subclasses, and adds a C API entrypoint esp_modem_set_transmit_hooks() (with opaque user_ctx) plus host tests validating hook firing, ordering, and clearing.

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[david-cermak]

@euripedesrocha Here's my workaround to get atomic load/store with shared_prt<> work across all IDF versions. (atomic_load/store_explicit vs. shared_ptr<>.load is either deprecated or not present)

Perhaps you have a better idea/suggestion how to address this?