system_setup: system setup API for Flutter UI part

This completes TODO "provide an API for this for the flutter UI to know
about it"

API is at /meta/client_variant with GET method, returns a string

subiquity/common/apidef.py

@@ -78,6 +78,8 @@ class API:
         class client_variant:
             def POST(variant: str) -> None:
                 """Choose the install variant - desktop/server"""
+            def GET() -> str:
+                """Get the install variant - desktop/server"""
 
         class confirm:
             def POST(tty: str) -> None:

subiquity/models/subiquity.py

@@ -105,6 +105,8 @@ class SubiquityModel:
         if root != '/':
             self.target = root
 
+        self.client_variant = ''
+
         self.debconf_selections = DebconfSelectionsModel()
         self.filesystem = FilesystemModel()
         self.identity = IdentityModel()
@@ -135,6 +137,7 @@ class SubiquityModel:
         self._postinstall_event = asyncio.Event()
 
     def set_source_variant(self, variant):
+        self.client_variant = variant
         self._cur_install_model_names = \
             self._install_model_names.for_variant(variant)
         self._cur_postinstall_model_names = \

subiquity/server/server.py

@@ -124,6 +124,9 @@ class MetaController:
         else:
             self.app.base_model.set_source_variant(variant)
 
+    async def client_variant_GET(self) -> str:
+        return self.app.base_model.client_variant
+
     async def ssh_info_GET(self) -> Optional[LiveSessionSSHInfo]:
         ips = []
         if self.app.base_model.network:

system_setup/client/client.py

@@ -40,9 +40,6 @@ class SystemSetupClient(SubiquityClient):
         ]
 
     def __init__(self, opts):
-        # TODO WSL:
-        # 3. provide an API for this for the flutter UI to know about it
-
         if is_reconfigure(opts.dry_run):
             self.variant = "wsl_configuration"
             self.controllers = [