Expand env value when possible

This commit enables users to pass environment variable as composition
of env variable such as:

```yaml
  - id: yolov5
    operator:
      outputs:
        - bbox
        - ready
      inputs:
        image: oasis_agent/image
        check: oasis_agent/yolov5_check
      python: ../../operators/yolov5_op.py
    env:
      YOLOV5_PATH: $HOME/Documents/dependencies/yolov5
      YOLOV5_WEIGHT_PATH: $DORA_DEP_HOME/dependencies/yolov5/yolov5n.pt
```

Note that this is only for the env field. Expanding env variable for
source path might conflict with using url as source path.

libraries/core/Cargo.toml

@@ -14,3 +14,4 @@ once_cell = "1.13.0"
 zenoh-config = { git = "https://github.com/eclipse-zenoh/zenoh.git", rev = "79a136e4fd90b11ff5d775ced981af53c4f1071b" }
 which = "4.3.0"
 uuid = { version = "1.2.1", features = ["serde"] }
+serde-with-expand-env = "1.1.0"

libraries/core/src/descriptor/mod.rs

@@ -1,6 +1,7 @@
 use crate::config::{CommunicationConfig, DataId, InputMapping, NodeId, NodeRunConfig, OperatorId};
 use eyre::{bail, Result};
 use serde::{Deserialize, Serialize};
+use serde_with_expand_env::with_expand_envs;
 use std::{
     collections::{BTreeMap, BTreeSet, HashMap},
     env::consts::EXE_EXTENSION,
@@ -222,8 +223,11 @@ pub struct CustomNode {
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(untagged)]
 pub enum EnvValue {
+    #[serde(deserialize_with = "with_expand_envs")]
     Bool(bool),
+    #[serde(deserialize_with = "with_expand_envs")]
     Integer(u64),
+    #[serde(deserialize_with = "with_expand_envs")]
     String(String),
 }