Support for multi-engine support

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Currently, only Wasmtime engine is supported.

We might want to support others like:

• WasmEdge
• WebAssembly Micro Runtime (WAMR)
• Wasmer
• Etc.

This way, we allow the developer to take advantage of the specific Wasm engine's unique features.

Ideally, a new directive such as WasmEngine <engine_name> could be used for each module that has to be loaded. This enables a quick way to test a module with one engine or another.

One relevant aspect to be considered is to have a kind of engine-feature matrix, so in case a WasmFeatureX is not supported by a specific engine, then maybe Apache shouldn't boot up.