fix(did-provider-pkh): refactor and simplify did:pkh plugin

[dchagastelles]

What issue is this PR fixing

There's no issue being fixed, just small improvements on did-provider-pkh plugin:

• minor parameters corrections
• use of namespace parameter
• allow passing privateKey to import key directly on identifier creation

What is being changed

Only changing did-provider-pkh plugin.

Quality

Check all that apply:

• I want these changes to be integrated
• I successfully ran pnpm i, pnpm build, pnpm test, pnpm test:browser locally.
• I allow my PR to be updated by the reviewers (to speed up the review process).
• I added unit tests.
• I added integration tests. [were already there]
• I did not add automated tests because _________, and I am aware that a PR without tests will likely get rejected.

[codecov]

Codecov Report

Base: 85.13% // Head: 84.97% // Decreases project coverage by -0.16% ⚠️

Coverage data is based on head (4dd14b9) compared to base (60bc5fd).
Patch coverage: 60.81% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #1113      +/-   ##
==========================================
- Coverage   85.13%   84.97%   -0.16%     
==========================================
  Files         140      140              
  Lines       14024    14050      +26     
  Branches     1422     1428       +6     
==========================================
  Hits        11939    11939              
- Misses       2085     2111      +26     

Impacted Files	Coverage Δ
packages/did-provider-pkh/src/pkh-did-provider.ts	62.72% <58.55%> (-14.32%)	⬇️
packages/did-provider-pkh/src/resolver.ts	80.95% <67.56%> (+4.86%)	⬆️

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[mirceanis]

Thanks for contributing this.

Since this is meant to clean up some code, I also made some notes that should make this align better with the patterns we use in other parts of the codebase.
It's mostly about error messages and debug output. Please take a look.

[mirceanis]

Thanks for the updates.
I suggested some more tiny fixes.