feat: forced tool tips (#8752)

Force an announcement tooltip in the CLI. This query the gh repo on this
[file](https://raw.githubusercontent.com/openai/codex/main/announcement_tip.toml)
which contains announcements in TOML looking like this:
```
# Example announcement tips for Codex TUI.
# Each [[announcements]] entry is evaluated in order; the last matching one is shown.
# Dates are UTC, formatted as YYYY-MM-DD. The from_date is inclusive and the to_date is exclusive.
# version_regex matches against the CLI version (env!("CARGO_PKG_VERSION")); omit to apply to all versions.
# target_app specify which app should display the announcement (cli, vsce, ...).

[[announcements]]
content = "Welcome to Codex! Check out the new onboarding flow."
from_date = "2024-10-01"
to_date = "2024-10-15"
version_regex = "^0\\.0\\.0$"
target_app = "cli"
``` 

To make this efficient, the announcement is queried on a best effort
basis at the launch of the CLI (no refresh made after this).
This is done in an async way and we display the announcement (with 100%
probability) iff the announcement is available, the cache is correctly
warmed and there is a matching announcement (matching is recomputed for
each new session).

codex-rs/tui/src/history_cell.rs

@@ -816,11 +816,11 @@ pub(crate) fn padded_emoji(emoji: &str) -> String {
 
 #[derive(Debug)]
 struct TooltipHistoryCell {
-    tip: &'static str,
+    tip: String,
 }
 
 impl TooltipHistoryCell {
-    fn new(tip: &'static str) -> Self {
+    fn new(tip: String) -> Self {
         Self { tip }
     }
 }