Add tls_client example (#305)

* Add tls_client example

This requires pico_mbedtls in the pico-sdk
Connects to worldtimeapi.org and retrieves a web page
Originally written by Floris Bos @maxnet

pico_w/tls_client/CMakeLists.txt

@@ -0,0 +1,44 @@
+add_executable(picow_tls_client_background
+        picow_tls_client.c
+        )
+target_compile_definitions(picow_tls_client_background PRIVATE
+        WIFI_SSID=\"${WIFI_SSID}\"
+        WIFI_PASSWORD=\"${WIFI_PASSWORD}\"
+        )
+target_include_directories(picow_tls_client_background PRIVATE
+        ${CMAKE_CURRENT_LIST_DIR}
+        ${CMAKE_CURRENT_LIST_DIR}/.. # for our common lwipopts
+        )
+target_link_libraries(picow_tls_client_background
+        pico_cyw43_arch_lwip_threadsafe_background
+        pico_lwip_mbedtls
+        pico_mbedtls
+        pico_stdlib
+        )
+pico_add_extra_outputs(picow_tls_client_background)
+
+add_executable(picow_tls_client_poll
+        picow_tls_client.c
+        )
+target_compile_definitions(picow_tls_client_poll PRIVATE
+        WIFI_SSID=\"${WIFI_SSID}\"
+        WIFI_PASSWORD=\"${WIFI_PASSWORD}\"
+        )
+target_include_directories(picow_tls_client_poll PRIVATE
+        ${CMAKE_CURRENT_LIST_DIR}
+        ${CMAKE_CURRENT_LIST_DIR}/.. # for our common lwipopts
+        )
+target_link_libraries(picow_tls_client_poll
+        pico_cyw43_arch_lwip_poll
+        pico_lwip_mbedtls
+        pico_mbedtls
+        pico_stdlib
+        )
+pico_add_extra_outputs(picow_tls_client_poll)
+
+# Ignore warnings from lwip code
+set_source_files_properties(
+        ${PICO_LWIP_PATH}/src/apps/altcp_tls/altcp_tls_mbedtls.c
+        PROPERTIES
+        COMPILE_OPTIONS "-Wno-unused-result"
+        )