Add comment for future EnvTestConfig API privatization (#1752)

### What
Add a comment to `EnvTestConfig` noting that the next time a field is
added, which requires a breaking change, the fields should be made
private and settable via builder-style functions.

### Why
`EnvTestConfig` has public fields, which makes adding new fields a
breaking change due to struct literal construction and exhaustive
pattern matching. The refactor is deferred to the next unavoidable
breaking change to avoid introducing one unnecessarily, and the comment
ensures the opportunity is not missed.

Close #1419

Author: leighmcculloch

soroban-sdk/src/env.rs

@@ -270,6 +270,9 @@ pub struct EnvTestConfig {
     /// JSON file to be written to disk when the Env is no longer referenced.
     /// Defaults to true.
     pub capture_snapshot_at_drop: bool,
+    // NOTE: Next time a field needs to be added to EnvTestConfig it will be a breaking change,
+    // take the opportunity to make the current field private, new fields private, and settable via
+    // functions. Why: So that it is the last time a breaking change is needed to the type.
 }
 
 #[cfg(any(test, feature = "testutils"))]