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

---------

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

firmware/common/flex_defs.hpp

@@ -0,0 +1,34 @@
+#ifndef __FLEX_DEFS_H__
+#define __FLEX_DEFS_H__
+
+#include <cstdint>
+#include <array>
+#include "baseband.hpp"
+
+namespace flex {
+
+enum class FlexMode : uint8_t {
+    FLEX_1600_2FSK,
+    FLEX_3200_2FSK,
+    FLEX_3200_4FSK,
+    FLEX_6400_4FSK
+};
+
+struct FlexStats {
+    uint32_t symbols_processed;
+    uint32_t total_frames;
+    uint32_t correct_frames;
+};
+
+struct FlexPacket {
+    uint32_t bitrate;  // 1600, 3200, 6400
+    uint32_t capcode;
+    uint32_t function;  // 0-3
+    uint32_t type;      // Message type (e.g. ALN, NUM, etc - could use enum)
+    char message[128];  // Decoded message text
+    uint32_t status;    // 0=OK, other=Errors
+};
+
+} /* namespace flex */
+
+#endif /*__FLEX_DEFS_H__*/