feat!: add ability to create plugins without an existing Context

[zshipko]

EIP: extism/proposals#8

This PR makes minor breaking changes to several SDKs, but not to runtime C API. The threadsafety updates in the Rust SDK are kind of specific to Rust, I'm not sure if it makes sense to add the locks to all the other SDKs at this point. For the most part the Context and Plugin types in the SDKs should be safe to use protected by a mutex but they aren't inherently threadsafe. That kind of locking should probably be done by the user.

• Runtime
  • improve thread safety
  • reinstantiates less
  • fixes a potential resource exhaustion bug from re-instantiating using the same store too many times
• Rust SDK
  • adds Send and Sync implementations for Context
  • adds test sharing a context between threads
  • adds Plugin::call_map to call a plugin and handle the output with the lock held
  • adds testing sharing an Arc<Mutex<Plugin>> between threads
  • adds Plugin::create and Plugin::create_from_manifest to create a plugin without a Context
• Python
  • BREAKING
  • changes Plugin constructor to take context as an optional named argument, to update use Plugin(data, context=context) instead
• Ruby
  • BREAKING
  • changes Plugin constructor to take context as an optional named argument, to update use Plugin.new(data, context=context) instead
• Go
  • adds NewPlugin and NewPluginFromManifest functions
• Node
  • BREAKING
  • changes Plugin constructor to take context as an optional named argument, to update use new Plugin(data, wasi, config, host, context) instead of new Plugin(context, data, wasi, functions, config) (most people are probably using context.plugin instead of the Plugin constructor anyway)
• OCaml
  • BREAKING
  • changes Plugin.create and Plugin.of_manifest to take context as an optional named argument, to update Plugin.create ~context data and Plugin.of_manifest ~context data instead
• Haskell
  • adds createPlugin and createPluginFromManifest functions
• Elixir
  • adds Plugin.new to make a plugin without going through Context.new_plugin
• Java
  • adds new Plugin constructors without a Context argument
• C++
  • BREAKING
  • Updates Plugin constructor to take an optional context as the last argument, instead of requiring it to be the first argument
  • Use Plugin(wasm, wasi, functions, ctx) instead of Plugin(ctx, wasm, wasi, functions)
• Zig
  • Adds Plugin.create and Plugin.createWithManifest to create plugins in their own context.