feat: Expand the DID scalar regex to accommodate did:pkh

[mcclurejt]

Description

did:pkh is a widely used method and should be captured by the regex

(issue #1800)

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Added test cases for a did:pkh string

Checklist:

• I have followed the CONTRIBUTING doc and the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests and linter rules pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[ardatan]

Thank you!