fix: Memory leak caused by openFile and osOpen functions

[oliveromahony]

Proposed changes

The parse function was leaking memory. On analysis of the code it looked like the fileOpen function was keeping the memory active, after the file was being used.

• Changed the io.Reader to io.ReadCloser in Open signature
• Changed the io.Reader to io.ReadCloser in osOpen signature
• Added defer to close the file after function exit in Parse

A snapshot of the memory in the NGINX Agent before these changes:

and after:

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md)

[ornj]

Can you also remove this type assertion block? I think it is no longer needed now that the correct type is passed.