fix(graphql): Checkpwd func and val inside eq func now work in custom dql

[Schartey]

The checkpwd func does not work in custom dql queries since #7775 . The query rewriter for custom dql did not handle the checkpwd function. Also it did not allow to use val inside of the eq function.

Example:

type Query {
    authenticate(username: String, email: String, password: String!): [User] @custom(dql: """
	query q($username: string, $email: string, $password: string) {
          var(func: type(User)) @filter(eq(User.username, $username) OR eq(User.email, $email)) {
              check as checkpwd(User.password, $password)
          }
              
          authenticate(func: eq(val(check), 1)) {
              id: uid
              username: User.username
              email: User.email
          }
      }
    """
    )
}

This query uses checkpwd in the first query, which would be rewritten to

check as User.password)

and it uses func: eq(val(check), 1) in the second query, which would result in

func: eq(check, "1")

I added handling for both of these issues.
The issue has also been documented in discuss

Please let me know if I missed something. I would be glad if this could make it into the next release, as it is a minor change.

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[MichelDiz]

@Schartey Master doesn't exist anymore. Can you open a new PR pointing to Main?
Closing and waiting a New PR.

Thank you.