feat(wasmtime) expose cache_config directive

[hishamhm]

This adds support for wasmtime { flag cache_config_load <config.toml>; }, for enabling disk caching of ahead-of-time compilation of .wasm files.

This should alleviate noticeable CPU spikes when starting up workers using Wasmtime and Wasm modules larger than a few megabytes.

Implementation notes:

• Added error checking to the config directive, even though not all wasmtime config checks do, because this one reports various errors such as file not found and parse errors in the config file.
• Kept the same name cache_config_load as documented in https://docs.wasmtime.dev/api/wasmtime/struct.Config.html#method.cache_config_load since that is the reference we point to in our documentation.

[hishamhm]

@thibaultcha the tests for cache enabled and disabled don't actually check that the cache was actually created (I don't know how to check for the existence of files at the end of a test within the Test::Nginx DSL), but if you run each of them with -- ONLY you can see that one of them creates the cache in the temporary dir and the other one doesn't.

[codecov]

Codecov Report

Merging #536 (fa89090) into main (bb139e1) will decrease coverage by 0.00210%.
Report is 2 commits behind head on main.
The diff coverage is 89.47368%.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main        #536         +/-   ##
===================================================
- Coverage   90.06471%   90.06261%   -0.00210%     
===================================================
  Files             47          47                 
  Lines          10045       10063         +18     
===================================================
+ Hits            9047        9063         +16     
- Misses           998        1000          +2     

Files	Coverage Δ
src/wasm/ngx_wasm_core_module.c	92.64706% <ø> (ø)
src/wasm/wrt/ngx_wrt_wasmtime.c	82.56881% <89.47368%> (+0.36816%)	⬆️

Flag	Coverage Δ
unit	90.12555% <86.66667%> (-0.00588%)	⬇️
valgrind	78.05435% <26.31579%> (-0.10945%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[thibaultcha]

Ultimately it's fine if all we test is our directive and its calling of the underlying wasmtime function. We aren't responsible for testing the behavior of the runtime itself, even though that's a plus if we can, but well...

[thibaultcha]

Do we need this in this file?

[thibaultcha]

Also looks unused