Introduce Python code generator for OpenAPI spec to dataclasses

Add a [Python code generator][1] that takes an OpenAPI definition and
outputs the corresponding data model as [dataclasses][2]

This is intended to be used in the Remote Asset Library project, to
create, download, parse, and validate information of a remote asset
library.

[1]: https://koxudaxi.github.io/datamodel-code-generator/
[2]: https://docs.python.org/3/library/dataclasses.html

## Running the Generator

The generator is a Python script, which creates its own Python
virtualenv, installs the dependencies it needs, and then runs the
generator within that virtualenv.

The script is intended to run via the `generate_datamodels` CMake
target. For example, `ninja generate_datamodels` in the build
directory.

## Details

The virtualenv is created in Blender's build directory, and is not
cleaned up after running. This means that subsequent runs will just
use it directly, instead of reinstalling dependencies on every run.

## Generated Code & Interaction with Build System

It is my intention that the code generation _only_ happens when the
OpenAPI specification changes. This means that the generated code will
be committed to Git like any hand-written code. Building Blender will
therefore _not_ require the code generator to run. Only people working
on the area that uses the generated code will have to deal with this.

Pull Request: https://projects.blender.org/blender/blender/pulls/139495

build_files/cmake/generate_datamodels.cmake

@@ -0,0 +1,13 @@
+# SPDX-FileCopyrightText: 2025 Blender Authors
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+set(GENERATE_DATAMODELS_SCRIPT "${CMAKE_SOURCE_DIR}/tools/utils/make_generate_datamodels.py")
+
+add_custom_target(generate_datamodels
+    COMMAND ${PYTHON_EXECUTABLE} ${GENERATE_DATAMODELS_SCRIPT} ${CMAKE_SOURCE_DIR}
+    DEPENDS ${GENERATE_DATAMODELS_SCRIPT}
+    WORKING_DIRECTORY ${CMAKE_BINARY_DIR}
+    COMMENT "Generating datamodels"
+    VERBATIM
+)