Add support for customToken

[tuananhnguyen-ct]

Fixes #157

[tuananhnguyen-ct]

@SaaldjorMike @jespermbyskov anyone can have a quick look please?

[fjerlov-cs]

This would be a breaking change for Go API users.

Could we rename it to AddV2 and call it from ingest_tokens_add.go and Add() to keep the signature? @SaaldjorMike

func (i *IngestTokens) Add(repositoryName string, tokenName string, parser string) (*IngestToken, error) {
	return i.AddV2(repositoryName, tokenName, parser, "")
}

[SaaldjorMike]

The most recent changes refactors how we interact with GraphQL for the humioctl tool. Going forward, you have to

1. update https://github.com/humio/cli/tree/master/internal/api/humiographql with any new interaction with the LogScale GraphQL API
2. generate Go-code based on the updated GraphQL queries/mutations: go generate ./...
3. update humioctl within https://github.com/humio/cli/tree/master/cmd/humioctl so it uses the newly generated code

Right now the code path is as follows: https://github.com/humio/cli/tree/master/cmd/humioctl calls https://github.com/humio/cli/tree/master/internal/api which in turn calls the autogenerated code in https://github.com/humio/cli/blob/master/internal/api/humiographql/humiographql.go.