feat(esp_partition): Add esp_partition_flash_binary() CMake function

Add a new CMake function esp_partition_flash_binary() that provides a
unified API for registering partition data binaries to be flashed. It
replaces the direct esptool_py_flash_target calls scattered across
components (spiffs, fatfs, nvs_flash) with a single function that:

- Resolves partition offset from the partition table automatically
- Determines encryption requirements (auto-detect or ALWAYS_PLAINTEXT)
- Creates per-partition flash targets (e.g. idf.py <partition>-flash)
- Optionally includes the binary in `idf.py flash` via FLASH_IN_PROJECT

On the linux target, the function registers binaries for pre-loading
into the emulated flash. A build-time manifest (linux_flash_data.txt)
is generated via file(GENERATE), and partition_linux.c reads it at
runtime to copy each binary into the memory-mapped flash buffer at
the correct offset.

The partition_ops example is updated to use the new function and
includes a custom_partition with pre-built data to demonstrate the
full workflow, including on the linux target.

tools/test_apps/build_system/custom_partition_subtypes/CMakeLists.txt

@@ -25,6 +25,7 @@ add_custom_target(blank_bin ALL DEPENDS ${blank_file})
 idf_build_get_property(target IDF_TARGET)
 
 if(NOT ${target} STREQUAL "linux")
-    add_dependencies(flash blank_bin)
-    esptool_py_flash_to_partition(flash "${partition}" "${blank_file}")
+    esp_partition_register_target("${partition}" "${blank_file}"
+        DEPENDS blank_bin
+        FLASH_IN_PROJECT)
 endif()