CMake: Support for XML documentation

Add new function generate_doc_source to python_callouts.cmake
Update Test extension to use generate_doc_source function and include generated file in the build.

Cleanup:
Fix godotcpp.py imports after rebase
Pre-Commit hook sorted python imports in doc_source_generator.py
- replace ${CMAKE_CURRENT_SOURCE_DIR} with ${godot-cpp_SOURCE_DIR} when referencing current working directory and script locations when invoking python scripts.

Co-authored-by: David Snopek <dsnopek@gmail.com>

doc_source_generator.py

@@ -0,0 +1,55 @@
+#!/usr/bin/env python
+
+import glob
+import os
+import zlib
+
+
+def generate_doc_source(dst, source):
+    g = open(dst, "w", encoding="utf-8")
+    buf = ""
+    docbegin = ""
+    docend = ""
+    for src in source:
+        src_path = str(src)
+        if not src_path.endswith(".xml"):
+            continue
+        with open(src_path, "r", encoding="utf-8") as f:
+            content = f.read()
+        buf += content
+
+    buf = (docbegin + buf + docend).encode("utf-8")
+    decomp_size = len(buf)
+
+    # Use maximum zlib compression level to further reduce file size
+    # (at the cost of initial build times).
+    buf = zlib.compress(buf, zlib.Z_BEST_COMPRESSION)
+
+    g.write("/* THIS FILE IS GENERATED DO NOT EDIT */\n")
+    g.write("\n")
+    g.write("#include <godot_cpp/godot.hpp>\n")
+    g.write("\n")
+
+    g.write('static const char *_doc_data_hash = "' + str(hash(buf)) + '";\n')
+    g.write("static const int _doc_data_uncompressed_size = " + str(decomp_size) + ";\n")
+    g.write("static const int _doc_data_compressed_size = " + str(len(buf)) + ";\n")
+    g.write("static const unsigned char _doc_data_compressed[] = {\n")
+    for i in range(len(buf)):
+        g.write("\t" + str(buf[i]) + ",\n")
+    g.write("};\n")
+    g.write("\n")
+
+    g.write(
+        "static godot::internal::DocDataRegistration _doc_data_registration(_doc_data_hash, _doc_data_uncompressed_size, _doc_data_compressed_size, _doc_data_compressed);\n"
+    )
+    g.write("\n")
+
+    g.close()
+
+
+def scons_generate_doc_source(target, source, env):
+    generate_doc_source(str(target[0]), source)
+
+
+def generate_doc_source_from_directory(target, directory):
+    generate_doc_source(target, glob.glob(os.path.join(directory, "*.xml")))