refactor: rename getRM for multiRM to more intuitive name

[amandavialva01]

Ticket

DET-10289

Description

Currently, getRM only returns the name of the desired resource manager, rather than the resource manager itself. Rename this to getRMName so that getRM can be implemented to retrieve the desired resource manager, as its function name suggests.

Ref here, where we use getRM to return the resource manager needed to provision resources within its corresponding cluster.

Test Plan

CI passes.

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]

Codecov Report

Attention: Patch coverage is 86.95652% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 45.14%. Comparing base (c3901c8) to head (4cb3bb7).
Report is 23 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9350      +/-   ##
==========================================
- Coverage   45.16%   45.14%   -0.03%     
==========================================
  Files        1230     1230              
  Lines      154567   154567              
  Branches     2405     2405              
==========================================
- Hits        69817    69782      -35     
- Misses      84555    84590      +35     
  Partials      195      195              

Flag	Coverage Δ
backend	41.75% <86.95%> (-0.07%)	⬇️
harness	64.07% <ø> (ø)
web	36.06% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
master/internal/rm/multirm/multirm.go	83.33% <86.95%> (ø)

... and 5 files with indirect coverage changes