Host SDK: add some host-side functions missing from the kernel.

[evacchi]

Adding a few missing host functions, sketching a "nice" API for end users at the same time.

We introduce ExtismHostFunction as an abstraction over Chicory's native HostFunction, taking a CurrentPlugin reference instead of a bare Instance. CurrentPlugin exposes some methods in Plugin and the new HostEnv.
HostEnv wraps and exposes functions from the Kernel and defines its own, namely log, config and var.

To simulate name-spacing, HostEnv define inner classes to HostEnv, that cannot be instantiated from the outside, so that we get a nice "fluent" API to invoke them; e.g.:

        var f = ExtismHostFunction.of("myfunc", List.of(), List.of(),
                (CurrentPlugin p, Value... args) -> {
                    p.log().log(LogLevel.INFO, "hello world");
                    return null;
        });

I defined some tests to go along with these to show them. Will add some more tests and/or together with #10 and #9.

[evacchi]

I have already integrated this locally to #10 and it seems fine.

[bhelx]

Just a side note, should we consider convenience methods for the levels? hostEnv.log().info("hello world");

[evacchi]

💯

[evacchi]

will do in a follow-up!