Added heterogenous job support

[nvjullin]

Heterogenous job allows multiple segments, used for more flexible scheduling and to allow multi-rack deployments.
For simplicity, this PR only splits jobs into one segment each for prefill and decode, instead of one per worker or more elaborate setups.
Use by setting het_jobs: true in resources, default false.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.65248% with 16 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@20f6cde). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/srtctl/cli/mixins/telemetry_stage.py	63.15%	7 Missing ⚠️
src/srtctl/core/runtime.py	89.18%	4 Missing ⚠️
src/srtctl/core/config.py	66.66%	2 Missing ⚠️
src/srtctl/core/slurm.py	89.47%	2 Missing ⚠️
src/srtctl/cli/do_sweep.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #178   +/-   ##
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  Coverage        ?   65.72%           
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  Files           ?       67           
  Lines           ?     8386           
  Branches        ?        0           
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  Hits            ?     5512           
  Misses          ?     2874           
  Partials        ?        0           

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