Add setters on `DidDocument`, remove `DidDocumentBuilder`

[Patrik-Stas]

Removes custom DidDocumentBuilder in favor of setters on DidDocument itself.

Rationale:

• While it's nice to have chaining API, the builder pattern didn't bring much in this case. All fields of DidDocument except for id are optional, so the builder itself didn't do much in terms of assuring DidDocument being valid. Rather then playing typical role of builder, assuring correctness of constructed datastructure, DidDocumentBuilder was rather used in manner of signalling "mut DidDoc". You could had very well rename DidDocumentBuilder to MutableDidDocument and the code would had been still making sense. This made implementation somewhat awkward where DidDocument had to be converted to DidDocumentBuilder in order to be edited (eg. DidPeerResolver), then subsequently converted back to DidDoc after edit.
• Additionally, with builder being self-consuming, it was difficult to use builder within iteration scopes.

It's worth noting that the mutating method could in future performs runtime checks, such as whether a provided DidUrl reference (eg pub fn add_authentication_ref(&mut self, reference: DidUrl)) is referring to existing VerificationMethod.

Additionally:

• Analogous approach applied on DidPeer4ConstructionDidDoc

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 189 lines in your changes are missing coverage. Please review.

Project coverage is 0.05%. Comparing base (413b183) to head (d3a02ef).

Files	Patch %	Lines
...peer_did/numalgos/numalgo4/construction_did_doc.rs	0.00%	114 Missing :warning:
did_core/did_doc/src/schema/did_doc.rs	0.00%	41 Missing :warning:
...did_peer/src/peer_did/numalgos/numalgo2/helpers.rs	0.00%	19 Missing :warning:
...rc/protocols/did_exchange/state_machine/helpers.rs	0.00%	4 Missing :warning:
...d_methods/did_resolver_sov/src/resolution/utils.rs	0.00%	4 Missing :warning:
did_core/did_methods/did_peer/src/resolver/mod.rs	0.00%	3 Missing :warning:
...ods/did_peer/src/peer_did/numalgos/numalgo2/mod.rs	0.00%	2 Missing :warning:
...ods/did_peer/src/peer_did/numalgos/numalgo4/mod.rs	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1164 +/- ## ======================================== - Coverage 0.05% 0.05% -0.01% ======================================== Files 484 484 Lines 24165 24184 +19 Branches 4489 4484 -5 ======================================== Hits 13 13 - Misses 24152 24171 +19 ``` | [Flag](https://app.codecov.io/gh/hyperledger/aries-vcx/pull/1164/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=hyperledger) | Coverage Δ | | |---|---|---| | [unittests-aries-vcx](https://app.codecov.io/gh/hyperledger/aries-vcx/pull/1164/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=hyperledger) | `0.05% <0.00%> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=hyperledger#carryforward-flags-in-the-pull-request-comment) to find out more.

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