Support non-persistent configuration with DELTACHAT_* env #3986

This way we can test some recently added config options that we don't want to expose in UI like
DeleteToTrash or SignUnencrypted. Note that persistent config options like DeleteToTrash should
remain anyway because they allow fine-grained (per-account) control. Having them matters for tests
also.

src/config.rs

@@ -1,5 +1,6 @@
 //! # Key-value configuration management.
 
+use std::env;
 use std::str::FromStr;
 
 use anyhow::{ensure, Context as _, Result};
@@ -317,6 +318,11 @@ impl Context {
 
     /// Get a configuration key. Returns `None` if no value is set, and no default value found.
     pub async fn get_config(&self, key: Config) -> Result<Option<String>> {
+        let env_key = format!("DELTACHAT_{}", key.as_ref().to_uppercase());
+        if let Ok(value) = env::var(env_key) {
+            return Ok(Some(value));
+        }
+
         let value = match key {
             Config::Selfavatar => {
                 let rel_path = self.sql.get_raw_config(key.as_ref()).await?;