Wasm: Wire up Wasm language module support to the config system.

This exposes various WebAssembly language module specific options.

The application type is "wasm".

There is a "module" option that is required, this specifies the full
path to the WebAssembly module to be run. This module should be in
binary format, i.e a .wasm file.

There are also currently eight function handlers that can be specified.
Three of them are _required_

1) request_handler

The main driving function. This may be called multiple times for a
single HTTP request if the request is larger than the shared memory.

2) malloc_handler

Used to allocate a chunk of memory at language module startup. This
memory is allocated from the WASM modules address space and is what is
sued for communicating between the WASM module (the guest) and Unit (the
host).

3) free_handler

Used to free the memory from above at language module shutdown.

Then there are the following five _optional_ handlers

1) module_init_handler

If set, called at language module startup.

2) module_end_handler

If set, called at language module shutdown.

3) request_init_handler

If set, called at the start of request. Called only once per HTTP
request.

4) request_end_handler

If set, called once all of a request has been sent to the WASM module.

5) response_end_handler

If set, called at the end of a request, once the WASM module has sent
all its headers and data.

Example config

  "applications": {
      "luw-echo-request": {
          "type": "wasm",
          "module": "/path/to/unit-wasm/examples/c/luw-echo-request.wasm",
          "request_handler": "luw_request_handler",
          "malloc_handler": "luw_malloc_handler",
          "free_handler": "luw_free_handler",
          "module_init_handler": "luw_module_init_handler",
          "module_end_handler": "luw_module_end_handler",
      }
  }

Reviewed-by: Alejandro Colomar <alx@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

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