Fixes #5669 - Enable chatCompletions experimental feature via CLI and ENV

https://www.meilisearch.com/docs/reference/api/chats#chat-completions-workspace-object
Currently the new `chatCompletions` feature could be enabled only via
```
curl \
  -X PATCH 'MEILISEARCH_URL/experimental-features/' \
  -H 'Content-Type: application/json' \
  --data-binary '{
    "chatCompletions": true
  }'
```

Now it will be possible to use
1) command line argument: `--experimental_chat_completions`
2) or environment variable:
   `export MEILI_EXPERIMENTAL_CHAT_COMPLETIONS=true`

Signed-off-by: Martin Tzvetanov Grigorov <mgrigorov@apache.org>

crates/index-scheduler/src/lib.rs

@@ -45,6 +45,9 @@ use std::path::{Path, PathBuf};
 use std::sync::{Arc, RwLock};
 use std::time::Duration;
 
+use crate::features::FeatureData;
+use crate::index_mapper::IndexMapper;
+use crate::utils::clamp_to_page_size;
 use dump::Dump;
 pub use error::Error;
 pub use features::RoFeatures;
@@ -71,9 +74,6 @@ use scheduler::Scheduler;
 use time::OffsetDateTime;
 use versioning::Versioning;
 
-use crate::index_mapper::IndexMapper;
-use crate::utils::clamp_to_page_size;
-
 pub(crate) type BEI128 = I128<BE>;
 
 const TASK_SCHEDULER_SIZE_THRESHOLD_PERCENT_INT: u64 = 40;
@@ -136,6 +136,8 @@ pub struct IndexSchedulerOptions {
     pub embedding_cache_cap: usize,
     /// Snapshot compaction status.
     pub experimental_no_snapshot_compaction: bool,
+    /// Whether the chat completions are enabled or not.
+    pub experimental_chat_completions: bool,
 }
 
 /// Structure which holds meilisearch's indexes and schedules the tasks
@@ -273,7 +275,7 @@ impl IndexScheduler {
                 .open(&options.tasks_path)
         }?;
 
-        // We **must** starts by upgrading the version because it'll also upgrade the required database before we can open them
+        // We **must** start by upgrading the version because it'll also upgrade the required database before we can open them
         let version = versioning::Versioning::new(&env, from_db_version)?;
 
         let mut wtxn = env.write_txn()?;
@@ -281,8 +283,11 @@ impl IndexScheduler {
         let queue = Queue::new(&env, &mut wtxn, &options)?;
         let index_mapper = IndexMapper::new(&env, &mut wtxn, &options, budget)?;
         let chat_settings = env.create_database(&mut wtxn, Some(CHAT_SETTINGS_DB_NAME))?;
+
         wtxn.commit()?;
 
+        configure_experimental_features(&env, &features, &options)?;
+
         // allow unreachable_code to get rids of the warning in the case of a test build.
         let this = Self {
             processing_tasks: Arc::new(RwLock::new(ProcessingTasks::new())),
@@ -922,6 +927,22 @@ impl IndexScheduler {
     }
 }
 
+fn configure_experimental_features(
+    env: &Env<WithoutTls>,
+    features: &FeatureData,
+    options: &IndexSchedulerOptions,
+) -> Result<()> {
+    let current_features = features.features().runtime_features();
+
+    let new_features = RuntimeTogglableFeatures {
+        chat_completions: options.experimental_chat_completions,
+        ..current_features
+    };
+
+    let wtxn = env.write_txn().map_err(Error::HeedTransaction)?;
+    features.put_runtime_features(wtxn, new_features)
+}
+
 /// The outcome of calling the [`IndexScheduler::tick`] function.
 pub enum TickOutcome {
     /// The scheduler should immediately attempt another `tick`.