Add Observe SDK support

[bhelx]

This is different than extism/extism#470

Because we don't rely on libextism in this library, I believe we should be able to add Observe support independently.

[neuronicnobody]

Wait for Observe SDK breaking changes to land . Ping @chrisdickinson

[chrisdickinson]

In order to accomplish this, we need to:

• Add a stage "between" createPlugin -> plugin and plugin.call() where the module is compiled but not instantiated
  • This is semi-related to Allow Initialization of Plugin from WebAssembly.Module #9. Today, all manifest items are resolved to ArrayBuffer bytes, but both Allow Initialization of Plugin from WebAssembly.Module #9 and this work mean we'll want to resolve manifest items down to WebAssembly.Module instances. (IOW: this makes a strong case for starting on Allow Initialization of Plugin from WebAssembly.Module #9 before 1.0)
• Add a plugin method for accessing WebAssembly.Module.customSections() for a sub-module of a plugin.
• Maybe add a PluginOptions hook for preinstantiate so the observe-sdk can intercept the module's customdata / exports before the module is instantiated (& optionally add additional host functions?)