cardano-rpc | Encode coin as String in JavaScript

[carbolymer]

Changelog

- description: |
    <insert-changelog-description-here>
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

Because protobuf-javascript encodes integers in protobuf as number, it can cause precision loss and number in addition to that is a floating point type. To work around that one has to annotate the proto fields with [jstype = JS_BIGINT]. Related issues:

• [JS] toString on proto.google.protobuf.Int64Value doesn't return the correct value on large numbers protocolbuffers/protobuf-javascript#74 (comment)
• JavaScript Bug - deserializeBinary uint64 field to a unsafe int and not correct number protocolbuffers/protobuf-javascript#67

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff

[carbolymer]

Update PR after the design is decided in

• fix(cardano): use string in JavaScript for cardano coin representation utxorpc/spec#166

and it is merged.