fix(ffi): prevent fatal error if there is the connection error during client creation

[erka]

Sooner or later this should be fixed.

thread '<unnamed>' panicked at flipt-engine-ffi/src/lib.rs:205:74:
called `Result::unwrap()` on an `Err` value: Server("failed to make request: error sending request for url (http://localhost:8080/internal/v1/evaluation/snapshot/namespace/default)")
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5
SIGABRT: abort

The idea is to start with empty state and use replace_snapshot logic to fill it. If the service isn't available the error will be returned until the service is live and state is updated.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 84.26%. Comparing base (3f7831c) to head (dbd591a).

Files	Patch %	Lines
flipt-engine-ffi/src/evaluator/mod.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #204      +/-   ##
==========================================
- Coverage   84.28%   84.26%   -0.03%     
==========================================
  Files          10       10              
  Lines        3049     3050       +1     
==========================================
  Hits         2570     2570              
- Misses        479      480       +1     

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[markphelps]

great idea

[erka]

related to #148