feat(external-node): external node distributed operation mode (#1457)

## What ❔

Adds a way to run External Node in a "distributed setup": that if there
is a need to launch different components of the EN on different machines
for performance reasons. The currently supported component split is
* `core`
* `tree`
* `tree_api` (depends on `tree`)
* `api` (may be split into two components: `http_api` and `ws_api`)

Example setup:

```
zk external-node -- --components=core
```

```
 env EN_HEALTHCHECK_PORT=3082 EN_PROMETHEUS_PORT=3323 zk external-node -- --components="tree,tree_api"
```
## Why ❔

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are the principles behind them? -->
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## Checklist

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- [ ] PR title corresponds to the body of PR (we generate changelog
entries from PRs).
- [ ] Tests for the changes have been added / updated.
- [ ] Documentation comments have been added / updated.
- [ ] Code has been formatted via `zk fmt` and `zk lint`.
- [ ] Spellcheck has been run via `zk spellcheck`.
- [ ] Linkcheck has been run via `zk linkcheck`.

core/bin/external_node/src/config/mod.rs

@@ -261,6 +261,19 @@ pub struct OptionalENConfig {
     pub l1_batch_commit_data_generator_mode: L1BatchCommitDataGeneratorMode,
 }
 
+#[derive(Debug, Clone, PartialEq, Deserialize)]
+pub struct ApiComponentConfig {
+    /// Address of the tree API used by this EN in case it does not have a
+    /// local tree component running and in this case needs to send requests
+    /// to some external tree API.
+    pub tree_api_url: Option<String>,
+}
+
+#[derive(Debug, Clone, PartialEq, Deserialize)]
+pub struct TreeComponentConfig {
+    pub api_port: Option<u16>,
+}
+
 impl OptionalENConfig {
     const fn default_filters_limit() -> usize {
         10_000
@@ -554,6 +567,8 @@ pub struct ExternalNodeConfig {
     pub optional: OptionalENConfig,
     pub remote: RemoteENConfig,
     pub consensus: Option<consensus::Config>,
+    pub api_component: ApiComponentConfig,
+    pub tree_component: TreeComponentConfig,
 }
 
 impl ExternalNodeConfig {
@@ -568,6 +583,14 @@ impl ExternalNodeConfig {
             .from_env::<OptionalENConfig>()
             .context("could not load external node config")?;
 
+        let api_component_config = envy::prefixed("EN_API")
+            .from_env::<ApiComponentConfig>()
+            .context("could not load external node config")?;
+
+        let tree_component_config = envy::prefixed("EN_TREE")
+            .from_env::<TreeComponentConfig>()
+            .context("could not load external node config")?;
+
         let client = HttpClientBuilder::default()
             .build(required.main_node_url()?)
             .expect("Unable to build HTTP client for main node");
@@ -619,6 +642,8 @@ impl ExternalNodeConfig {
             required,
             optional,
             consensus: read_consensus_config().context("read_consensus_config()")?,
+            tree_component: tree_component_config,
+            api_component: api_component_config,
         })
     }
 }