FLEX pager app (#2883)

* Add FLEX pager support

- Introduced a new FLEX configuration function in baseband_api.
- Added FLEX application view and associated UI elements.
- Implemented FLEX processing logic in proc_flex, including demodulation and message handling.
- Updated CMakeLists to include new FLEX source files and headers.
- Enhanced message system to support FLEX-specific messages and statistics.

This commit lays the groundwork for FLEX pager functionality, allowing for the reception and processing of FLEX messages.

* Fixed baseband and moved app to external with some other fixes.

* Format code

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Co-authored-by: RocketGod <57732082+RocketGod-git@users.noreply.github.com>

firmware/application/baseband_api.cpp

@@ -328,6 +328,11 @@ void set_noaaapt_config() {
     send_message(&message);
 }
 
+void set_flex_config() {
+    const FlexConfigureMessage message{};
+    send_message(&message);
+}
+
 void set_siggen_tone(const uint32_t tone) {
     const SigGenToneMessage message{
         TONES_F2D(tone, TONES_SAMPLERATE)};