Refactor backend code

[maia-iyer]

Refactors backend code by splitting two files into five.

Before, the structure followed within /api/agent:

• server.go
  • Contained definition of server type
  • Contained configuration parsing and plugin initialization logic
  • Contained Handlers for all main functions
• api.go
  • Contained SPIRE API implementation
  • Contained Tornjak API implementation

Revised structure gives a file to each of these concerns:

• server.go: contains definition of server type
• config.go: contains config parsing and plugin initialization logic
• handlers.go: contains handlers for all main functions
• spire_apis.go: contains SPIRE API implementations
• tornjak_apis.go: contains Tornjak API implementations

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Additionally to pass the CodeQL warning, I revised the code here to check for file traversal attack AFTER joining paths instead of BEFORE joining paths.