🍭 example: more generalized examples

[gaukas]

In this PR we combined a few contributions:

• Make examples more generalized. Now dialer, listener, and relay take an input wasm file to run.
• Fix an issue causing contexts not being properly propagated into the underlying WebAssembly runtime. Deprecate most of context-less functions (i.e., Instead of NewCore, recommend using NewCoreWithContext.)
• Make intentional sleeps in some test shorter to speed up testing. Also, reduce the hardcoded test iteration.

Note that we are not bumping up the dependency, gaukas/wazero to v1.6.2-w yet. More comprehensive testing to be done on it to better understand its performance implication.

[erikziyunchi]

LGTM!