WASM plugin support

[bastjan]

This PR allows registering and running WASM plugins from OCI registries like ghcr.io.

There is currently supoprt for context plugins.

The plugin lifecycle is managed separately from the ManagedResource. For each plugin a runner pool is spawned. The repository is continously monitored for new versions and runners are replaced transparently if a new version exists.

The code on this branch can be tested by:

kubectl apply -k config/crd
go run ./main.go controller --enable-dynamic-admission-webhook=false --alpha-plugins=true

TODOs

• Better data passing interfaces. We might want to go with WASM reactor mode and pass data using protobuf or json.
  • Plugin global conf/ better conf from managed resource/ Retrospection from plugin?
• Trigger and output variants of plugins
  • Small supervisor for the trigger plugin.
• Better docs/how-tos
  • Rust and Go sample plugins
• Plugin versioning interface.
  The plugin should export a fucntion espejote_plugin_version and espejote_plugin_capabilities.
  OR maybe better: Configure using a "custom section" which would not violate spec by calling a function before _initialize.
• More metrics (compile time, run time, pool usage, ...)
• PullPolicies? Always, IfNotPresent, Never?

Bonus TODOs

• Implement some form of virtual file system in stealthrocket/wasi-go for passing /etc/resolv.conf and the TLS certificates.
  They are open for contributions Name resolution does not work without filesystem access dispatchrun/wasi-go#82
• The current pool implementation rotates runners as such that all capacity is filled which is a bit wasteful. Update the implementation or dynamically scale capacity.

Checklist

• Categorize the PR by setting a good title and adding one of the labels:
  bug, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog
• Update tests.