[shell] Add api to get path to the sidecar's binary

[FabianLars]

This comes up once in a while and should be easy to add (it's also quite easy to do in user's rust code but a built-in api would still be nice)

[animeshchaudhri]

@FabianLars Hey can I work on this?

[FabianLars]

sure, much appreciated!

[animeshchaudhri]

@FabianLars Any similar pr that can help me out ?

[FabianLars]

hmmm, tricky. This seems to be the most recent one that simply adds a new api to a plugin https://github.com/tauri-apps/plugins-workspace/pull/2095/files (didn't find anything for the shell plugin)
and to get the sidecar path i think this is the starting point

plugins-workspace/plugins/shell/src/commands.rs

Lines 115 to 131 in 6b4c391

	let mut command = if options.sidecar {
	let program = PathBuf::from(program);
	let program_as_string = program.display().to_string();
	let program_no_ext_as_string = program.with_extension("").display().to_string();
	let configured_sidecar = window
	.config()
	.bundle
	.external_bin
	.as_ref()
	.and_then(|bins| {
	bins.iter()
	.find(|b| b == &&program_as_string || b == &&program_no_ext_as_string)
	})
	.cloned();
	if let Some(sidecar) = configured_sidecar {
	scope.prepare_sidecar(&program.to_string_lossy(), &sidecar, args)?
	} else {