fix: Enable values with an = sign in default_resources (#2340)

### Description

In the default_resources block in a profile, variables cannot contain =
signs; this is, however, needed to specify slurm_extra (e.g.,
--mail-type=none). The reason is in
https://github.com/snakemake/snakemake/blob/main/snakemake/resources.py#L23:
the argument is split by the = sign; this leads to an error if the
result contains more than 2 elements (which occurs when there is more
than one = sign).

Closes #2290 

### QC
<!-- Make sure that you can tick the boxes below. -->

* [x] The PR contains a test case for the changes or the changes are
already covered by an existing test case.
* [x] The documentation (`docs/`) is updated to reflect the changes or
this is not necessary (e.g. if the change does neither modify the
language nor the behavior or functionalities of Snakemake).

---------

Co-authored-by: Johannes Köster <johannes.koester@uni-due.de>
Co-authored-by: Johannes Köster <johannes.koester@tu-dortmund.de>

snakemake/resources.py

@@ -20,7 +20,7 @@ class DefaultResources:
     @classmethod
     def decode_arg(cls, arg):
         try:
-            return arg.split("=")
+            return arg.split("=", maxsplit=1)
         except ValueError:
             raise ValueError("Resources have to be defined as name=value pairs.")
 

tests/test_slurm.py

@@ -80,3 +80,20 @@ def test_slurm_complex():
             ]
         ),
     )
+
+
+@skip_on_windows
+def test_slurm_extra_arguments():
+    """Make sure arguments to default resources
+    are allowed to contain = signs, which is needed
+    for extra slurm arguments"""
+    run(
+        dpath("test_slurm_mpi"),
+        slurm=True,
+        show_failed_logs=True,
+        use_conda=True,
+        default_resources=DefaultResources(
+            ["slurm_account=runner", "slurm_partition=debug",
+             "slurm_extra='--mail-type=none'"]
+        ),
+    )