Add stubs for "gdb" (#8013)

This commit adds type stubs for the "gdb" package, the Python API to
extend GDB (https://sourceware.org/gdb/onlinedocs/gdb/Python-API.html).

stubs/gdb/METADATA.toml

@@ -0,0 +1,12 @@
+version = "12.1.*"
+extra_description = """\
+  Type hints for GDB's \
+  [Python API](https://sourceware.org/gdb/onlinedocs/gdb/Python-API.html). \
+  Note that this API is available only when running Python scripts under GDB: \
+  is is not possible to install the `gdb` package separately, for instance \
+  using `pip`.\
+"""
+
+# Since the "gdb" Python package is available only inside GDB, it is not
+# possible to install it through pip, so stub tests cannot install it.
+stubtest = false