deps: update dependency flagsmith/sdk-spec to v0.9.0 - #82

…es (#83)

Lets Rust applications run experiments with Flagsmith. An application
can get an identity's experiment flag, which records which variant the
identity was exposed to. It can also send tracking events, such as
conversions, to Flagsmith's events API. This implements SDK spec
[v0.7.1](https://github.com/Flagsmith/sdk-spec/releases/tag/v0.7.1), and
its spec run now includes the `events` scenarios.

## Changes
- [x] Events can be enabled on the client (`enable_events`). Events are
buffered and sent to Flagsmith's events API in batches: on a timer, when
the buffer fills, on `flush_events`, and when the client is dropped. The
events API URL, flush interval and buffer size can be configured.
Offline clients send no events.
- [x] `get_experiment_flag` returns an identity's flag, and requires an
identifier. When the identity is enrolled in a running experiment, it
also records a `$flag_exposure` event for the variant served, with the
identity's traits.
- [x] `track_event` records an application event, with an optional
identity, value and metadata. Names starting with `$` are reserved.
- [x] `track_exposure_event` records an exposure for a variant evaluated
elsewhere. It requires an identity.
- [x] Flags from remote identity evaluation carry the `variant` served
and the running `experiment`.
- [x] Updates the SDK spec to v0.7.1, from Renovate's #82, and runs its
`core`, `events` and `local-evaluation` scenarios. Empty string flag
values, new in v0.7.1, already worked.

Contributes to Flagsmith/sdk-spec#13

Review effort: 3/5

---------

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: flagsmith-engineering[bot] <216534828+flagsmith-engineering[bot]@users.noreply.github.com>
Co-authored-by: flagsmithsmith[bot] <346993984+flagsmithsmith[bot]@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>