set custom "User-Agent" for ExApps (#329)

Fixes #327

Changes proposed in this pull request:

* Added a custom header in the format `ExApp/test-deploy/1.1.0
(httpx/0.24.1)` to help "system administrators" identify which requests
are taking a long time to complete.

nc_py_api/_session.py

@@ -12,6 +12,7 @@
 from os import environ
 
 from httpx import AsyncClient, Client, Headers, Limits, ReadTimeout, Request, Response
+from httpx import __version__ as httpx_version
 from starlette.requests import HTTPConnection
 
 from . import options
@@ -511,6 +512,7 @@ def _create_adapter(self, dav: bool = False) -> AsyncClient | Client:
                 "AA-VERSION": self.cfg.aa_version,
                 "EX-APP-ID": self.cfg.app_name,
                 "EX-APP-VERSION": self.cfg.app_version,
+                "user-agent": f"ExApp/{self.cfg.app_name}/{self.cfg.app_version} (httpx/{httpx_version})",
             },
         )
 
@@ -535,6 +537,7 @@ def _create_adapter(self, dav: bool = False) -> AsyncClient | Client:
                 "AA-VERSION": self.cfg.aa_version,
                 "EX-APP-ID": self.cfg.app_name,
                 "EX-APP-VERSION": self.cfg.app_version,
+                "User-Agent": f"ExApp/{self.cfg.app_name}/{self.cfg.app_version} (httpx/{httpx_version})",
             },
         )