shim: organize code to different modules

this commit organized code to different modules:
- constants: to handling constants across entire crate
- engine: the execution engine
- source: manages application sources
- stdio_hook: a trigger hook for stdio
- trigger: manages spin triggers
- utils: utility functions

it also removes ResolvedAppSource as it's not needed

Signed-off-by: jiaxiao zhou <jiazho@microsoft.com>

containerd-shim-spin/src/constants.rs

@@ -0,0 +1,22 @@
+/// SPIN_ADDR_DEFAULT is the default address and port that the Spin HTTP trigger
+/// listens on.
+pub(crate) const SPIN_ADDR_DEFAULT: &str = "0.0.0.0:80";
+/// SPIN_HTTP_LISTEN_ADDR_ENV is the environment variable that can be used to
+/// override the default address and port that the Spin HTTP trigger listens on.
+pub(crate) const SPIN_HTTP_LISTEN_ADDR_ENV: &str = "SPIN_HTTP_LISTEN_ADDR";
+/// RUNTIME_CONFIG_PATH specifies the expected location and name of the runtime
+/// config for a Spin application. The runtime config should be loaded into the
+/// root `/` of the container.
+pub(crate) const RUNTIME_CONFIG_PATH: &str = "/runtime-config.toml";
+/// Describes an OCI layer with Wasm content
+pub(crate) const OCI_LAYER_MEDIA_TYPE_WASM: &str = "application/vnd.wasm.content.layer.v1+wasm";
+/// Expected location of the Spin manifest when loading from a file rather than
+/// an OCI image
+pub(crate) const SPIN_MANIFEST_FILE_PATH: &str = "/spin.toml";
+/// Known prefix for the Spin application variables environment variable
+/// provider: https://github.com/fermyon/spin/blob/436ad589237c02f7aa4693e984132808fd80b863/crates/variables/src/provider/env.rs#L9
+pub(crate) const SPIN_APPLICATION_VARIABLE_PREFIX: &str = "SPIN_VARIABLE";
+/// Working directory for Spin applications
+pub(crate) const SPIN_TRIGGER_WORKING_DIR: &str = "/";
+/// Name of the Spin lock file
+pub(crate) const SPIN_LOCK_FILE_NAME: &str = "spin.lock";