include neighboring envelope gitoids when searching by subject

[mikhailswift]

currently our api response for get by subject digest consists of a stream of these messages:

message GetBySubjectDigestResponse {
  string gitoid = 1;
  string collectionName = 2;
  repeated string attestations = 3;
}

we should include other subjects that appear within the in-toto statement in this response, as well as gitoids of other envelopes that share the subject. perhaps this may look something like

message GetBySubjectDigestResponse {
  string gitoid = 1;
  string collectionName = 2;
  repeated string attestations = 3;
  repeated SubjectDetails subjects = 4;
}

message SubjectDetails {
    string subject = 1;
    repeated Digest digests = 2;
    repeated GetBySubjectDigestResponse edges = 3;
}

SubjectDetails.edges would only be present on the parent's subjects to prevent an infinite loop, and shouldn't include the current envelope being described by the parent response nor the current subject being searched since any envelopes with the searched subject will appear in the stream of responses.

Example:

envelope1
subjects: shared-subject, shared-subject2, unique-subject
collectionName: clone

envelope2
subjects: shared-subject, shared-subject2, unique-subject2
collectionName: build

result when searching for "shared-subject" should look something like

[
{
  gitoid = envelope1
  collectionName = clone
  attestations = [....]
  subjects = [
  {
    subject: shared-subject2
    digests: [....]
    edges: [
    {
      gitoid = envelope2
      collectionName = build
      attestations = [....]
    }, {
    subject: unique-subject
    digests: [....]
    edges: []
  }
  ]
}, {
  gitoid = envelope2
  collectionName = build
  attestations = [....]
  subjects = [
  {
    subject: shared-subject2
    digests: [....]
    edges: [
    {
      gitoid = envelope1
      collectionName = clone
      attestations = [....]
     }, {
    subject: unique-subject2
    digests: [....]
    edges: []
    }
    ]
  ]
}
]