refactor: rename filter_rel_fields to base_related_field_filters

[dpgaspar]

SUMMARY

Small refactor to improve the current confusing names for declaring related filters on ModelRestApi

Renames: filter_rel_fields to base_related_field_filters

currently we have:
filter_rel_fields: That declared a base filter that is always applied on the declared related fields
related_field_filters: That applies a filter on related fields

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #22508 (dd67fc5) into master (7d8fff8) will decrease coverage by 11.10%.
The diff coverage is 90.00%.

❗ Current head dd67fc5 differs from pull request most recent head 0557ba4. Consider uploading reports for the commit 0557ba4 to get more accurate results

@@             Coverage Diff             @@
##           master   #22508       +/-   ##
===========================================
- Coverage   66.90%   55.79%   -11.11%     
===========================================
  Files        1851     1851               
  Lines       70696    70696               
  Branches     7764     7764               
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- Hits        47299    39447     -7852     
- Misses      21375    29227     +7852     
  Partials     2022     2022               

Flag	Coverage Δ
hive	52.46% <90.00%> (ø)
mysql	?
postgres	?
presto	52.36% <90.00%> (ø)
python	57.92% <90.00%> (-23.33%)	⬇️
sqlite	?
unit	51.14% <90.00%> (ø)

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

Impacted Files	Coverage Δ
superset/views/filters.py	70.83% <ø> (-29.17%)	⬇️
superset/views/base_api.py	61.47% <66.66%> (-36.97%)	⬇️
superset/charts/api.py	50.37% <100.00%> (-35.83%)	⬇️
superset/dashboards/api.py	51.14% <100.00%> (-41.43%)	⬇️
superset/datasets/api.py	49.18% <100.00%> (-38.22%)	⬇️
superset/queries/api.py	100.00% <100.00%> (ø)
superset/queries/saved_queries/api.py	61.11% <100.00%> (-33.34%)	⬇️
superset/reports/api.py	58.95% <100.00%> (-31.35%)	⬇️
superset/utils/dashboard_import_export.py	0.00% <0.00%> (-100.00%)	⬇️
superset/tags/core.py	4.54% <0.00%> (-95.46%)	⬇️
... and 289 more

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[villebro]

LGTM with an optional comment on moving part of the docstring.

[villebro]

Should we move this comment to related_field_filters and say there that the filter is applied in addition (or on top, whichever sounds better) to base_related_field_filters?