introduce standard containerized variant

[rfbgo]

This defines a standard variant +containerized and enables it for the relevant workflow managers. Apps can use this for conditional vars:

eg

+    with when("+containerized"):
+        workload_variable(
+            "container_mounts",
+            default="{maxtext_mount}",
+            description="All container mounts in a ramble variable",
+            workloads=all_workloads,
+        )

The most important thing to review here is the use of is_containerized to enable it in the workflow manager classes, as it's kind of like a silent extension to the workflow manager language

[gemini-code-assist]

Code Review

This pull request introduces a reserved containerized variant to Ramble, defaulting to False. It allows applications to conditionally define container-related variables (such as mounts) using with when("+containerized"):. Workflow managers like gke-mpi and slurm-pyxis are marked as containerized (is_containerized = True), which automatically propagates the containerized=True variant to their associated applications unless overridden. Unit tests are also added to verify the reservation, validation, and propagation of this variant. I have no feedback to provide as there are no review comments.

[ramble-pr-bot]

Ramble Performance Test Metrics

Results produced with commit: d408072

Test Name	Outcome	Duration (s)	Most Recent Run (s)	Last 5 Avg (s)
test_analyze_large_file	passed	2.1087	2.1005 (de4ccce)	2.0984
test_large_template_expansion	passed	2.1291	1.6327 (de4ccce)	1.6532
test_many_experiments	passed	29.8159	24.6595 (de4ccce)	25.4157
test_many_objects_defaults	passed	16.7170	14.4773 (de4ccce)	14.4945
test_matrix_filter_perf	passed	1.3615	1.1632 (de4ccce)	1.1704

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.49%. Comparing base (de4ccce) to head (d408072).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1715      +/-   ##
===========================================
+ Coverage    93.48%   93.49%   +0.01%     
===========================================
  Files          367      367              
  Lines        35916    35977      +61     
===========================================
+ Hits         33575    33638      +63     
+ Misses        2341     2339       -2     

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