feat: Add SystemCalls and Interrupts support for AIX load metrics

[Dylan-M]

Description

Add support for SystemCalls() and Interrupts() functions for AIX platform to retrieve system calls and interrupt counts since boot.

Part 3 of splitting #1969: load package
Depends on #1979 #1980

Changes

• Add SystemCalls() and Interrupts() functions to load_aix.go that delegate to context-aware implementations
• Implement SystemCallsWithContext() and InterruptsWithContext() in load_aix_nocgo.go using vmstat command
• Add parseVmstatLine() helper function to parse vmstat output and extract context switches, interrupts, and syscalls
• Add getVmstatMetrics() helper function for shared metric extraction logic
• Implement test dependency injection via testInvoker and getInvoker() for better testability
• Update AvgWithContext() to use the new getInvoker() abstraction
• Improve MiscWithContext() to properly parse AIX process states (A, I, W, T, Z) and populate context switches from vmstat
• Update command invocations to use the new invoker abstraction for consistency

Testing

Changes have been validated with gofmt and golangci-lint.

[shirou]

As I commented on #1980 (comment), I'm going to close this PR.