feat: Add selection label to FlatRuns page

[thiagodallacqua-hpe]

Ticket: ET-309

Description

Add the selection label to the flat runs table.

Test Plan

• Enable the Flat runs feature
• go to a runs table
  • there should be a label with the total number of runs
• make a selection
  • the label should be updated accordingly

Checklist

• [ ] Changes have been manually QA'd
• [ ] User-facing API changes need the "User-facing API Change" label.
• [ ] Release notes should be added as a separate file under docs/release-notes/. See Release Note for details.
• [ ] Licenses should be included for new code which was copied and/or modified from any external code.

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 30 lines in your changes missing coverage. Please review.

Project coverage is 46.27%. Comparing base (8379b13) to head (f97962b). Report is 1 commits behind head on main.

:exclamation: Current head f97962b differs from pull request most recent head 3590c00

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HEAD has 9 uploads less than BASE

Flag	BASE (8379b13)	HEAD (f97962b)
harness	9	0

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9548      +/-   ##
==========================================
- Coverage   53.01%   46.27%   -6.75%     
==========================================
  Files        1256      929     -327     
  Lines      153021   123048   -29973     
  Branches     3240     3024     -216     
==========================================
- Hits        81129    56941   -24188     
+ Misses      71741    65949    -5792     
- Partials      151      158       +7     

Flag	Coverage Δ
harness	?
web	47.96% <0.00%> (-3.47%)	:arrow_down:

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Files	Coverage Δ
webui/react/src/pages/FlatRuns/FlatRuns.tsx	10.83% <0.00%> (+0.27%)	:arrow_up:

... and 406 files with indirect coverage changes

[johnkim-det]

closing and replacing with https://github.com/determined-ai/determined/pull/9670