feat(crypto): CRP-2597 Extend NiDkgTag with HighThresholdForKey variant

[fspreiss]

Extends the NiDkgTag with a new variant NiDkgTag::HighThresholdForKey that holds a MasterPublicKeyId.

Notes:

• Makes the representation for NiDkgTag explicit with #[repr(isize)] because a #[repr(inttype)] must be provided on an enum if it has a non-unit variant with a discriminant (E0732). The representation should be the same as it was before, given that isize is the default representation for enums.

  Note that casting NiDkgTag with as, e.g., to i32 or even isize is no longer possible because "an as expression can be used to convert enum types to numeric types only if the enum type is unit-only or field-less" (see Enumeration casting and E0605).

• The conversion impl TryFrom<i32> for NiDkgTag had to be removed, because it is no longer possible, because an i32 itself is no longer sufficient for instantiating an NiDkgTag.

  This conversion was used in two places: (1) impl TryFrom<NiDkgIdProto> for NiDkgId and (2) build_tagged_transcripts_map. There, the conversion is now implemented directly.

• No longer Implements EnumIter for NiDkgTag because this would require a Default implementation for MasterPublicKeyId, which does not make sense, at least in production. Instead implements EnumCount.

TODOs:

• Address the issue that //rs/tests/consensus/orchestrator:cup_compatibility_test fails
• Naming: NiDkgTag::HighThresholdForKey vs NiDkgTag::HighThresholdForKeyId
• use optional in new protobuf fields?: Rust struct definitions are the same
• decide how to deal with #[allow(clippy::result_large_err)] in crypto code
• add tests for changes in rs/crypto/src/sign/threshold_sig/store.rs