* ci: add makefile

- add a Makefile with basic build, test, clean, etc.
- simplify the Dockerfile by moving some scripts and conf files to directories
- update README
- allow easy setting of GOLANG version and PULSAR version for testing

Signed-off-by: Paul Gier <paul.gier@datastax.com>

* dockerfile: copy individual conf files

Copy individual config files instead of directory when building the Dockerfile to catch any missing/incorrect files.

Signed-off-by: Paul Gier <paul.gier@datastax.com>

* update apache pulsar link in readme

Signed-off-by: Paul Gier <paul.gier@datastax.com>

Signed-off-by: Paul Gier <paul.gier@datastax.com>

Makefiles are commonly used for building golang projects.

- add a Makefile with basic build, test, clean, etc.
- simplify the Dockerfile by moving some scripts and conf files to directories
- update README
- Allows easy overriding of pulsar version and golang version used during testing

### Motivation

Add a Makefile to make it easier to remember the commands for building and testing the project.  For example:
* `make build` instead of `go build ./pulsar`
* `make test` instead of `./docker-ci.sh`
* `make lint` instead of `golangci-lint run`