Marshal/unmarshal custom options in JSON and add image convert command

[bufdev]

This PR does two things:

1. Properly marshals/unmarshals custom options in JSON. This fixes Custom options are omitted from image json #50. For example:

syntax = "proto3";

import "google/protobuf/descriptor.proto";

extend google.protobuf.FieldOptions {
  int32 baz = 50007;
}

message Foo {
  string bar = 1 [(baz) = 42];
}

Running buf image build --exclude-imports --exclude-source-info -o -#format=json | jq results in:

{
  "file": [
    {
      "name": "a.proto",
      "dependency": [
        "google/protobuf/descriptor.proto"
      ],
      "messageType": [
        {
          "name": "Foo",
          "field": [
            {
              "name": "bar",
              "number": 1,
              "label": "LABEL_OPTIONAL",
              "type": "TYPE_STRING",
              "jsonName": "bar",
              "options": {
                "[baz]": 42
              }
            }
          ]
        }
      ],
      "extension": [
        {
          "name": "baz",
          "number": 50007,
          "label": "LABEL_OPTIONAL",
          "type": "TYPE_INT32",
          "extendee": ".google.protobuf.FieldOptions",
          "jsonName": "baz"
        }
      ],
      "syntax": "proto3"
    }
  ],
  "bufbuildImageExtension": {}
}

2. Adds buf experimental image convert, that takes an Image as input, and outputs an Image, allowing conversion to/from binary/JSON, and filtering of imports and/or source code info. This fixes Add functionality to convert Image formats #44.

For example, the following command:

$ buf image build -o image.bin

Will produce the same file as:

$ buf image build -o - | buf experimental image convert -i - -o -#format=json | buf experimental image convert -i -#format=json -o image.bin