[SCHEMA] Correlating existing validator issues to schema entries

[bendhouseart]

Adding existing code's from Validator Issues to entities in schema where REQUIRED if is defined.

e.g.

# where entity once looked like the following
MRIASLCommonMetadataFieldsM0Type:
    selectors:
    - modality == "MRI"
    - datatype == "perf"
    - suffix == "asl"
    - sidecar.M0Type == "Estimate"
    fields:
        M0Estimate: REQUIRED

It now contains a reference to the existing code in the validator:

MRIASLCommonMetadataFieldsM0Type:
    selectors:
    - modality == "MRI"
    - datatype == "perf"
    - suffix == "asl"
    - sidecar.M0Type == "Estimate"
    fields:
        M0Estimate:
            level: REQUIRED
            code:
                M0ESTIMATE_NOT_DEFINED: |
                You must define 'M0Estimate' for this file, in case 'M0Type' is defined as 'Estimate'. 
                'M0Estimate' is a single numerical whole-brain M0 value (referring to the M0 of blood), 
                only if obtained externally (for example retrieved from CSF in a separate measurement).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.50%. Comparing base (acb2d92) to head (c054ae5).

Additional details and impacted files

@@              Coverage Diff               @@
##           schema-sprint    #1063   +/-   ##
==============================================
  Coverage          71.50%   71.50%           
==============================================
  Files                  9        9           
  Lines                930      930           
==============================================
  Hits                 665      665           
  Misses               265      265           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.