clientinfo: thread CI_PURPOSE through FbClientInfo

Summary:
When the sandcastle worker sets the CI_PURPOSE environment variable, we want to pass that information to Mononoke. This diff adds a ci_purpose field to FbClientInfo, which is serialized into the X-Client-Info header similar to Sandcastle Alias.

Their values are not dissimlar form aliases but better suited for demand control. In faxct, they are the identifier RE uses for CI demand control. This table contain some examples: https://fburl.com/scuba/ci_demand_budget_entity_purpose_quotas_grodd_inde/kluni1nk
| purpose | Hits | Samples |
| -----| -----| ----- |
| "diff_citadel_fbcode" | 11 | 11 |
| "autoknowledge" | 8 | 8 |
| "citadel_orchestrator" | 8 | 8 |
| "ig-postcommit" | 6 | 6 |
| "rl_robotics_xqb" | 6 | 6 |
| "whatsapp_ml_agent" | 6 | 6 |
| "ads_ai_baf" | 5 | 5 |
| "devmate_code_review" | 5 | 5 |
| "quality_analyser" | 5 | 5 |
| "whatsapp-cf" | 5 | 5 |

If no purpose is set, the field is omitted from the header.

Reviewed By: muirdm

Differential Revision: D109559711

fbshipit-source-id: 52681b934573b224929558e7bb8a27eea5e2458f

Author: Clara Rull

eden/scm/lib/clientinfo/src/facebook.rs

@@ -34,6 +34,8 @@ pub struct FbClientInfo {
     #[serde(skip_serializing_if = "Option::is_none")]
     sandcastle_vcs: Option<String>,
     #[serde(skip_serializing_if = "Option::is_none")]
+    ci_purpose: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
     atlas: Option<bool>,
     #[serde(skip_serializing_if = "Option::is_none")]
     atlas_rl: Option<bool>,
@@ -68,6 +70,10 @@ impl FbClientInfo {
         self.sandcastle_vcs.as_deref()
     }
 
+    pub fn ci_purpose(&self) -> Option<&str> {
+        self.ci_purpose.as_deref()
+    }
+
     pub fn is_atlas(&self) -> Option<bool> {
         self.atlas
     }
@@ -121,6 +127,7 @@ pub fn get_fb_client_info() -> FbClientInfo {
         sandcastle_alias: var("SANDCASTLE_ALIAS").ok(),
         sandcastle_type: var("SANDCASTLE_TYPE").ok(),
         sandcastle_vcs: var("SANDCASTLE_VCS").ok(),
+        ci_purpose: var("CI_PURPOSE").ok(),
         atlas: atlas_env_flag("ATLAS"),
         atlas_rl: atlas_rl_from_whoami(),
         atlas_env_id: var("ATLAS_ENV_ID").ok(),

eden/scm/lib/clientinfo/src/oss.rs

@@ -40,6 +40,10 @@ impl FbClientInfo {
         None
     }
 
+    pub fn ci_purpose(&self) -> Option<&str> {
+        None
+    }
+
     pub fn is_atlas(&self) -> Option<bool> {
         None
     }