Add prefix-based inheritance to codegen

[GlassOfWhiskey]

This commit adds the possibility to specify a parent parser for each prefix in a Schema SALAD specification. The idea is that each extensions should come with a different namespace, and would define its own parser for its classes.

With this commit, a user can leverage the --codegen-parent option to explicitly map a prefix to a parent parser (e.g., a Python module) with a "prefix=package" syntax. For example:

--codegen-parent "https://w3id.org/cwl/salad=schema_salad.metaschema"

Inheritance is not able to automatically reconstruct the whole class hierarchy, meaning that if B inherits from A and C inherits from B, the user must manually specify the mapping also for namespace A. However, we do not expect huge parser hierarchies for schema SALAD specifications.

[mr-c]

In general, this looks good! Lets see this output of this PR in action before we merge & release

[codecov]

Codecov Report

❌ Patch coverage is 67.01031% with 192 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.58%. Comparing base (d48545d) to head (33f6420).

Files with missing lines	Patch %	Lines
src/schema_salad/runtime.py	68.23%	67 Missing and 14 partials ⚠️
src/schema_salad/python_codegen_support.py	0.00%	71 Missing ⚠️
src/schema_salad/metaschema.py	77.27%	37 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1028      +/-   ##
==========================================
+ Coverage   70.82%   71.58%   +0.76%     
==========================================
  Files          52       53       +1     
  Lines       10118     9912     -206     
  Branches     2119     2058      -61     
==========================================
- Hits         7166     7096      -70     
+ Misses       2386     2264     -122     
+ Partials      566      552      -14     

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