Refactor ServiceBuilder

[Patrik-Stas]

Builder refactoring

• Remove extra alias indirection ServiceTypeAlias - all its usage (2 places) fits on one screen

• Remove add_service_types in favor of add_service_type. The removed function was only used once and easily replacable by its "singular" version

• Remove ServiceBuilderWithServiceType - ServiceType is in fact optional, as https://sovrin-foundation.github.io/sovrin/spec/did-method-spec-template.html says:

  If the types field is present

  So requiring user to call one of the (originally 2 methods) is just not right. If we were to be just "opinionated" to require users of our builder to do so, it should follow same pattern as other required attributes such as id, service_endpoint, extra.

• If add_service_type is called with the same value as been provided before, throw InvalidInput instead of quietly doing nothing

• If add_service_type is called with empty string, throw InvalidInput error

Note:
Probably stating obvious, but I take it that ServiceBuilder was somewhat of a shortcut as we didn't intend to use builder macro libraries at the time, and building full blown state pattern builder by hand is cumbersome (hence why required attributes are part of ServiceBuilder<E>::new(....) constructor directly. Perhaps down the line we can adopt the same builder library as in messages crate.

Additional

Squashed some additional tweak, don't want create crate X pull requests so I'll squash further smaller improvements here.

• Implement Debug for Didso it prints Did { did: "did:foo:bar", method: Some("foo"), id: "bar" } instead of previously Did { did: "did:foo:bar", method: Some(4..7), id: 8..11 }
• Implement Display for DidDocument which prints the ddo as json

[codecov-commenter]

Codecov Report

Merging #1021 (0b2e21e) into main (1a414f0) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1021      +/-   ##
==========================================
- Coverage   36.31%   36.30%   -0.01%     
==========================================
  Files         386      386              
  Lines       22040    22044       +4     
  Branches     4062     4062              
==========================================
  Hits         8003     8003              
- Misses      11877    11881       +4     
  Partials     2160     2160              

Flag	Coverage Δ
unittests-aries-vcx	36.30% <0.00%> (-0.01%)	⬇️

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Files	Coverage Δ
did_parser/src/did.rs	0.00% <0.00%> (ø)