[MLIR] Split ExecutionEngine Initialization out of ctor into an explicit method call (#153373)

This PR introduces a mechanism to defer JIT engine initialization,
enabling registration of required symbols before global constructor
execution.

## Problem

Modules containing `gpu.module` generate global constructors (e.g.,
kernel load/unload) that execute *during* engine creation. This can
force premature symbol resolution, causing failures when:
- Symbols are registered via `mlirExecutionEngineRegisterSymbol` *after*
creation
- Global constructors exist (even if not directly using unresolved
symbols, e.g., an external function declaration)
   - GPU modules introduce mandatory binary loading logic

## Usage
```c
// Create engine without initialization
MlirExecutionEngine jit = mlirExecutionEngineCreate(...);

// Register required symbols
mlirExecutionEngineRegisterSymbol(jit, ...);

// Explicitly initialize (runs global constructors)
mlirExecutionEngineInitialize(jit);
```

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Co-authored-by: Mehdi Amini <joker.eph@gmail.com>