wasi-nn: add minimum serialization on WASINNContext

[yamt]

currently this is not necessary because context (WASINNContext) is local to instance. (wasm_module_instance_t)

i plan to make a context shared among instances in a cluster when fixing #4313. this is a preparation for that direction.

an obvious alternative is to tweak the module instance context APIs to allow declaring some kind of contexts instance-local. but i feel, in this particular case, it's more natural to make "wasi-nn handles" shared among threads within a "process".

note that, spec-wise, how wasi-nn behaves wrt threads is not defined at all because wasi officially doesn't have threads yet. i suppose, at this point, that how wasi-nn interacts with wasi-threads is something we need to define by ourselves, especially when we are using an outdated wasi-nn version.

with this change, if a thread attempts to access a context while another thread is using it, we simply make the operation fail with the "busy" error. this is intended for the mimimum serialization to avoid problems like crashes/leaks/etc. this is not intended to allow parallelism or such.

no functional changes are intended at this point yet.

cf.
#4313
#2430

[lum1n0us]

Just saying, I used to think you would prefer to use atomic variables to achieve similar functionality.

[yamt]

Just saying, I used to think you would prefer to use atomic variables to achieve similar functionality.

originally i wanted to use trylock. (and filed #4386)

direct uses of atomics would make it a bit difficult to read/maintain. it can be done later if/when it turns out desirable.