fix: extend_schema_field with dict param and oas 3.1

[codebutler]

the dict was being mutated, so every subsequent use of the field would have more stuff appended to the schema

without a fix, the test fails with this diff:

+++ 
@@ -42,12 +42,15 @@
           readOnly: true
         field2:
           type:
-          - string
+          - - string
+            - 'null'
           - 'null'
           readOnly: true
         field3:
           type:
-          - string
+          - - - string
+              - 'null'
+            - 'null'
           - 'null'
           readOnly: true
       required:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.58%. Comparing base (aeca119) to head (6f3a656).
Report is 78 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1147      +/-   ##
==========================================
- Coverage   98.63%   98.58%   -0.05%     
==========================================
  Files          71       72       +1     
  Lines        8711     8782      +71     
==========================================
+ Hits         8592     8658      +66     
- Misses        119      124       +5     

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