[feature] Adds a custom GraphQL directive to filter nested keys

[pxp928]

Is your feature request related to a problem? Please describe.
#1610 introduced the ability to filter results via contains and startswith but this does not work on nested fields such as subject.id, subject.algorithm or subject.digest in the query below:

query GetAllSLSA {

  HasSLSA (hasSLSASpec: {}) @filter(keyName: "subject.id", operation: CONTAINS, value: "710") {

  id

 subject {

    id

    algorithm

    digest

  }

  }

}

Describe the solution you'd like
Have the ability to query nested keys with contains and startsWith with custom GraphQL directive.

[pxp928]

FYI @kanchan-dhamane, created the issue to capture the update from PR #1610. Thank You!