feat: add StringAdapter to file adapter module

[sanbei101]

This pull request introduces a new StringAdapter to handle policy strings directly, along with necessary integrations and updates to the existing codebase. The most important changes include the addition of the StringAdapter struct and its implementation, updates to the adapter module to include the new adapter, and modifications to the main library file to use the new adapter.

Introduction of StringAdapter:

• src/adapter/file_adapter.rs: Added the StringAdapter struct with methods to load, save, and clear policies from a string, and implemented the Adapter trait for it.

Module updates:

• src/adapter/mod.rs: Updated to include StringAdapter in the list of exported adapters.

Library updates:

• src/lib.rs: Modified to conditionally export StringAdapter along with other adapters.

[github-actions]

Benchmark for c5a05d3

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Test	PR Benchmark	Master Benchmark	%
b_benchmark_rbac_model_large	10.3±0.10ms	10.4±0.12ms	-0.96%
benchmark priority model	4.3±0.08µs	4.3±0.08µs	0.00%
benchmark_abac_model	2.6±0.05µs	2.6±0.10µs	0.00%
benchmark_basic_model	3.9±0.07µs	3.9±0.04µs	0.00%
benchmark_key_match	7.3±0.15µs	7.2±0.09µs	+1.39%
benchmark_raw	0.0±0.00ns	0.0±0.00ns	NaN%
benchmark_rbac_model	6.5±0.15µs	6.2±0.06µs	+4.84%
benchmark_rbac_model_medium	910.7±11.30µs	914.7±14.55µs	-0.44%
benchmark_rbac_model_with_domains	6.1±0.10µs	6.1±0.11µs	0.00%
benchmark_rbac_with_deny	8.8±0.15µs	9.1±0.11µs	-3.30%
benchmark_rbac_with_resource_roles	4.5±0.11µs	4.5±0.09µs	0.00%
benchmark_role_manager_large	4.6±0.03ms	4.6±0.26ms	0.00%
benchmark_role_manager_medium	308.9±3.22µs	305.4±3.18µs	+1.15%
benchmark_role_manager_small	94.2±1.09µs	91.9±1.66µs	+2.50%