Better PyApp integration

[johannesloibl]

Hey, me again ;)

I'd like to request (and maybe even contribute) more integration options for PyApp.
Hatch right now only offers the scripts option, which is transforming your project scripts into a bunch of executables. While this may work for some easy use cases, it has following disadvantages:

• Only [project.scripts] is taken into account and [project.gui-scripts] is ignored.
  So if your project needs to build both, it is hard to configure different PyApp environment variables for both.
• No control over the naming of built executables. Right now the version number is appended and the script name is prepended. I'd like to make it template-able, something like mycustomname, mycustomname-{version} or {dist_name}-{version}.
• __main__.py cannot be built if script is left empty. Also other options PyApp provides (PYAPP_EXEC_*) are not supported.

Here an example TOML i could imagine:

[tool.hatch.build.targets.binary]

[tool.hatch.build.targets.binary.variables]
CARGO_TARGET_DIR = ".tmp/pyapp_cargo"
PYAPP_DISTRIBUTION_EMBED = "false"
PYAPP_FULL_ISOLATION = "true"
PYAPP_PIP_EXTRA_ARGS = "--index-url ..."
PYAPP_UV_ENABLED = "true"

[tool.hatch.build.targets.binary.build-targets]
# Build the CLI from myapp.cli:cli
myapp-cli = { name = "{dist_name}-cli.exe", variables = {PYAPP_IS_GUI = "false", PYAPP_EXEC_SPEC = "myapp.cli:cli"}}
# Build the GUI from myapp.__main__
myapp-gui = { name = "{dist_name}.exe", variables = {PYAPP_IS_GUI = "true", PYAPP_EXEC_MODULE = "myapp"}}

This would enable all possible options of PyApp within Hatch and integrate it better.
What do you think? If you like it i could contribute this feature.