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

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