Add Hash equal schema for GraphQL

[pxp928]

Add in the Hash Equal attestation for graphQL:

HashEqual:

• Description: attestation that the artifact is equal to another artifact (for example SHA1 and SHA256 represent the same artifact) with a justification.

• Subject: artifact

• Object: is_equal ( artifact )

• Properties: justification ( string )
  source ( string )
  collector ( string )

fragment allHashEqualTree on HashEqual {
  justification
  artifacts {
    algorithm
    digest
  }
  origin
  collector
}

query Q1 {
  HashEquals(hashEqualSpec: {}) {
    ...allHashEqualTree
  }
}

query Q2 {
  HashEquals(hashEqualSpec: {origin: "testing backend"}) {
    ...allHashEqualTree
  }
}

query Q3 {
  HashEquals(
    hashEqualSpec: {artifacts: [{algorithm: "sha1", digest: "7a8f47318e4676dacb0142afa0b83029cd7befd9"}]}
  ) {
    ...allHashEqualTree
  }
}

query Q4 {
  HashEquals(
    hashEqualSpec: {artifacts: [{algorithm: "sha256", digest: "89bb0da1891646e58eb3e6ed24f3a6fc3c8eb5a0d44824cba581dfa34a0450cf"}]}
  ) {
    ...allHashEqualTree
  }
}

query Q5 {
  HashEquals(
    hashEqualSpec: {artifacts: [{algorithm: "sha256", digest: "6bbb0da1891646e58eb3e6a63af3a6fc3c8eb5a0d44824cba581d2e14a0450cf"}]}
  ) {
    ...allHashEqualTree
  }
}

query Q6 {
  HashEquals(
    hashEqualSpec: {artifacts: [{algorithm: "sha512", digest: "374ab8f711235830769aa5f0b31ce9b72c5670074b34cb302cdafe3b606233ee92ee01e298e5701f15cc7087714cd9abd7ddb838a6e1206b3642de16d9fc9dd7"}]}
  ) {
    ...allHashEqualTree
  }
}

[pxp928]

@lumjjb, @mihaimaruseac, @mlieberman85 looking for feedback on improvements or missing features.

[pxp928]

Ready for review

[mlieberman85]

LGTM