Refactor GraphQL File Upload Implementation

[DimuthuMadushan]

Description:
When uploading a file using the ballerina GraphQL package, the multipart request should follow the GraphQL Multipart Form Request Specification. According to this, the map field values in multipart request, should be preceded by the word variables.
Ex:

curl http://sample.com \
    -F operations='{ "query": "mutation($file: Upload!) { fileUpload(file: $file) { link } }", "variables": { "file": null } }' \
    -F map='{ "0": ["variables.file"] }' \
    -F [email protected]

But the graphql package accept the following format:

curl http://sample.com \
    -F operations='{ "query": "mutation($file: Upload!) { fileUpload(file: $file) { link } }", "variables": { "file": null } }' \
    -F map='{ "0": ["file"] }' \
    -F [email protected]

here, -F map = {"0": [file]} should be -F map = {"0": [variables.file]}