codegen: generate client resource test and Terraform acceptance test

[maksym-nazarenko]

This PR introduces templates and codegen binary CLI flags to generate resource test files.

The flow to generate all files would be:

1. Generate MikroTik resource stub via downloading specs from remote RouterOS (Parse inspected console item definition #212)
2. Adjust the generated file with proper struct field types and tags
3. Generate client test with

   $ go run cmd/mikrotik-codegen/main.go mikrotik -src client/dns.go -test -name DnsRecord -dest client/dns_test.go

4. Generate Terraform resource with

   $ go run cmd/mikrotik-codegen/main.go terraform -src client/dns.go -dest mikrotik/resource_dns_test.go

5. Generate Terraform acceptance test

   $ go run cmd/mikrotik-codegen/main.go terraform -src client/dns.go -accTest -dest mikrotik/resource_dns_test.go