Support to take the user-defined test report template

[LinuxSuRen]

Currently, all the test reports are embed in the code lines. But it's quite normal that uesrs want a new test report style. A simple solution is that allowing pass a user-defined template.

A suggestion is adding a flag to the following command:

atest run --report-template (my-template.md | my-template.html | https://foo/my-template.html)

References

• https://github.com/LinuxSuRen/api-testing/blob/02d7e23cb322558e011e9992f108ce1afe3637b5/pkg/runner/writer_markdown.go#L33
• https://github.com/LinuxSuRen/api-testing/blob/02d7e23cb322558e011e9992f108ce1afe3637b5/pkg/runner/writer_html.go#L33