ExternalData: Add support for custom download scripts

Add support for a special URL template to map the fetch operation
to a project-specified .cmake script insead of using file(DOWNLOAD).

Extend the Module.ExternalData test to cover the behavior.
Extend the RunCMake.ExternalData test to cover error cases.

Tests/Module/ExternalData/CMakeLists.txt

@@ -10,7 +10,9 @@ if(NOT "${CMAKE_CURRENT_SOURCE_DIR}" MATCHES "^/")
 endif()
 set(ExternalData_URL_TEMPLATES
   "file://${slash}${CMAKE_CURRENT_SOURCE_DIR}/%(algo)/%(hash)"
+  "ExternalDataCustomScript://MyScript1/%(algo)/%(hash)"
   )
+set(ExternalData_CUSTOM_SCRIPT_MyScript1 "${CMAKE_CURRENT_SOURCE_DIR}/MyScript1.cmake")
 set(ExternalData_BINARY_ROOT "${CMAKE_CURRENT_BINARY_DIR}/ExternalData")
 file(REMOVE_RECURSE ${ExternalData_BINARY_ROOT}) # clean test
 
@@ -23,6 +25,7 @@ ExternalData_Add_Test(Data1
   COMMAND ${CMAKE_COMMAND}
     -D Data=DATA{Data.dat}
     ${Data1CheckSpaces}
+    -D DataScript=DATA{DataScript.dat}
     -D DataMissing=DATA{DataMissing.dat}
     -D DataMissingWithAssociated=DATA{DataMissing.dat,Data.dat}
     -D SeriesA=DATA{SeriesA.dat,:}

Tests/Module/ExternalData/Data1Check.cmake

@@ -8,6 +8,10 @@ if(DEFINED DataSpace)
     message(SEND_ERROR "Input file:\n  ${DataSpace}\ndoes not have expected content, but [[${lines}]]")
   endif()
 endif()
+file(STRINGS "${DataScript}" lines LIMIT_INPUT 1024)
+if(NOT "x${lines}" STREQUAL "xDataScript")
+  message(SEND_ERROR "Input file:\n  ${DataScript}\ndoes not have expected content, but [[${lines}]]")
+endif()
 if(DataMissing)
   if(EXISTS "${DataMissing}")
     message(SEND_ERROR

Tests/Module/ExternalData/DataScript.dat.md5

@@ -0,0 +1 @@
+fd95c03719e8626c0d10a818f9996dc5

Tests/Module/ExternalData/MyScript1.cmake

@@ -0,0 +1,5 @@
+if(ExternalData_CUSTOM_LOCATION STREQUAL "MD5/fd95c03719e8626c0d10a818f9996dc5")
+  file(WRITE "${ExternalData_CUSTOM_FILE}" "DataScript")
+else()
+  set(ExternalData_CUSTOM_ERROR "no ${ExternalData_CUSTOM_LOCATION} known")
+endif()