feat: make Memfault project key optional

 ### Summary

 In cases where an upstream device uploads data to Memfault on behalf
 of a downstream device, the project key is not required. This change
 allows the project key to be optional in those cases, which for the
 NCS Memfault sources is when we are not using MDS or the Memfault
 Zephyr HTTP client.

 ### Test Plan

 Confirmed that before this change, the build fails if the project key
 is not set for a generic cortex-m application where Memfault is enabled:

 ```
 west build -b qemu_cortex_m3 -p always zephyr/samples/hello_world -- \
 -DCONFIG_MEMFAULT=y \
 -DCONFIG_MEMFAULT_NCS_FW_TYPE=\"test-app\" \
 -DCONFIG_MEMFAULT_NCS_DEVICE_ID=\"my-test-device\"
 ```

 Confirmed after this change, the build succeeds with this same build command.

 Also confirmed that after this change, the build still fails if the project key
 is not set for projects where MDS or the Memfault Zephyr HTTP client
 are used:

  ```
west build --pristine always --board nrf52840dk/nrf52840 \
nrf/samples/bluetooth/peripheral_mds
  ...
  /Users/gminnehan/projects/memfault-sdk-nrf-workspace/zephyr/include/zephyr/toolchain/gcc.h:87:36: error: static assertion failed: "Memfault Project Key not configured. Please visit https://goto.memfault.com/create-key/nrf91 "
   87 | #define BUILD_ASSERT(EXPR, MSG...) _Static_assert((EXPR), "" MSG)
      |                                    ^~~~~~~~~~~~~~
/Users/gminnehan/projects/memfault-sdk-nrf-workspace/nrf/modules/memfault-firmware-sdk/memfault_integration.c:40:1: note: in expansion of macro 'BUILD_ASSERT'
   40 | BUILD_ASSERT(sizeof(CONFIG_MEMFAULT_NCS_PROJECT_KEY) > 1,
      | ^~~~~~~~~~~~
  ```

  ```
  west build -b thingy91/nrf9160/ns -p always \
  nrf/applications/asset_tracker_v2 -- \
  -DOVERLAY_CONFIG=overlay-memfault.conf
  ...
  /Users/gminnehan/projects/memfault-sdk-nrf-workspace/zephyr/include/zephyr/toolchain/gcc.h:87:36: error: static assertion failed: "Memfault Project Key not configured. Please visit https://goto.memfault.com/create-key/nrf91 "
   87 | #define BUILD_ASSERT(EXPR, MSG...) _Static_assert((EXPR), "" MSG)
      |                                    ^~~~~~~~~~~~~~
/Users/gminnehan/projects/memfault-sdk-nrf-workspace/nrf/modules/memfault-firmware-sdk/memfault_integration.c:40:1: note: in expansion of macro 'BUILD_ASSERT'
   40 | BUILD_ASSERT(sizeof(CONFIG_MEMFAULT_NCS_PROJECT_KEY) > 1,
      | ^~~~~~~~~~~~
  ```

Author: gminn

modules/memfault-firmware-sdk/memfault_integration.c

@@ -35,9 +35,11 @@ LOG_MODULE_REGISTER(memfault_ncs, CONFIG_MEMFAULT_NCS_LOG_LEVEL);
 #define MEMFAULT_URL	"https://goto.memfault.com/create-key/nrf"
 #endif
 
+#if defined(CONFIG_BT_MDS) || defined(CONFIG_MEMFAULT_HTTP_ENABLE)
 /* Project key check */
 BUILD_ASSERT(sizeof(CONFIG_MEMFAULT_NCS_PROJECT_KEY) > 1,
 	"Memfault Project Key not configured. Please visit " MEMFAULT_URL " ");
+#endif
 
 /* Firmware type check */
 BUILD_ASSERT(sizeof(CONFIG_MEMFAULT_NCS_FW_TYPE) > 1, "Firmware type must be configured");