feat: change DI VM purpose resolution

[mishasizov-SK]

Using ResolveSigningVMWithRelationship function causes a bug in VM relationship verification for Data integrity.

1. For instance, let's take issuer's DID, that after resolving has the following structure:

   {
   "verificationMethod": [
   ID: "abc",
   .....
   ],
   "authentication": [
   ID: "abc",
   .....
   ]
   }

2. Function docRes.DIDDocument.VerificationMethods() with empty arguments returns a map with 2 elements: key VerificationRelationshipGeneral and Authentication. Values for those keys are verification methods with the same ID field.
3. Because func docRes.DIDDocument.VerificationMethods() returns a map, the order of elements in for ... range loop is not predictable. It might be a case, when first element will be VerificationRelationshipGeneral.
4. In this case function verificationRelationshipName() returns ""
5. Then, condition

if rel == "" {
            rel = "assertionMethod"
        }

takes place and declares hardcoded relationship.

[codecov[bot]]

Codecov Report

Merging #3630 (82c6456) into main (717faac) will decrease coverage by 0.01%. The diff coverage is 82.19%.

@@            Coverage Diff             @@
##             main    #3630      +/-   ##
==========================================
- Coverage   86.93%   86.92%   -0.01%     
==========================================
  Files         367      367              
  Lines       50046    50098      +52     
==========================================
+ Hits        43507    43549      +42     
- Misses       4954     4961       +7     
- Partials     1585     1588       +3     

Files Changed	Coverage Δ
component/models/dataintegrity/signer.go	83.57% <81.15%> (+0.23%)	:arrow_up:
.../models/dataintegrity/suite/ecdsa2019/ecdsa2019.go	93.18% <100.00%> (-0.37%)	:arrow_down: