Add resource_minio_s3_object 'source' support for PutObject

Only 'content' and 'content_base64' have been implemented sofar, this PR adds 'source' support for resource_minio_s3_object, derived from https://github.com/hashicorp/terraform-provider-aws/blob/main/internal/service/s3/bucket_object.go#L388-L405

Reference

• Resolves: #552

Closing issues

• Closes: #552

Sorry for the delay in looking at this! It would be great to add some test cases here for CI if you have a chance

@pjsier Is there some sort of 'best practice' for testing opening/reading files in Go? I've only dabbled a bit in the language so far and I don't know much about writing unit tests in it.

@fhemberger I'm not sure on overall best practices, but I think we could leverage a similar pattern to the AWS provider adding a method to create tempfiles that can then be used later in the test

https://github.com/hashicorp/terraform-provider-aws/blob/43bd19db00807459b8c5ae3b4faaa91a183d3a7f/internal/service/s3/object_test.go#L702-L734

@pjsier I'm still having trouble setting up the unit tests, however, I'd really appreciate it if the fix could land anyways, as:

1. The source attribute is currently documented to be available, yet it's not implemented at all
2. There hasn't been a single unit test for minio_s3_object so far to build on.
3. The code is taken directly from Hashicorp's own official AWS provider.

So even without test cases, that's already an improvement over the current state.

/ping @pjsier @felladrin

Thanks @fhemberger! It's being released on v2.1.0.

@felladrin Thank you!