feat: add isolated instance

[toddbaert]

Adds spec for isolated instances.

Resolves: #359

Relevant work:

• feat: Add isolated (non-singleton) OpenFeature API instances js-sdk#1325
• feat: lock provider binding to a single API instance js-sdk#1342

[gemini-code-assist]

Summary of Changes

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This pull request enhances the OpenFeature specification by introducing the concept of isolated API instances. This change addresses advanced use cases such as micro-frontends, dependency injection, and testing scenarios, where independent API instances with fully isolated state are required. It defines how these instances should be created, behave, and manage their associated providers, moving beyond the default global singleton model.

Highlights

• Introduced Isolated API Instances: Added the concept of 'Isolated API Instances' to the OpenFeature specification, allowing for independent, non-singleton API instances with their own state.
• Defined Glossary Term: Included 'Isolated API Instance' in the specification's glossary, providing a clear definition for this new concept.
• Detailed Requirements for Isolated Instances: Added a new section (1.8) to the flag evaluation specification, outlining six requirements for isolated API instances, covering their creation, behavior, and provider management.

Changelog

• specification/glossary.md
  • Added 'Isolated API Instance' to the table of contents.
  • Included a detailed definition for 'Isolated API Instance'.
• specification/sections/01-flag-evaluation.md
  • Added a new section '1.8. Isolated API Instances' to the flag evaluation document.
  • Defined the need for isolated instances with examples like micro-frontends and testing.
  • Introduced Requirement 1.8.1 for a factory function to create independent API instances.
  • Specified Requirement 1.8.2 that isolated instances must conform to the same API contract.
  • Recommended in Requirement 1.8.3 that the factory function be housed in a distinct module for discoverability.
  • Stipulated in Requirement 1.8.4 that a provider instance must not be registered with more than one API instance simultaneously.
  • Required in Requirement 1.8.5 that the provider mutator must indicate an error if a provider is already bound.
  • Mandated in Requirement 1.8.6 that a provider must be unbound when no longer registered with an API instance.

Activity

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[gemini-code-assist]

Code Review

This pull request introduces the concept of "Isolated API Instances" to the OpenFeature specification. It includes a new glossary entry and a detailed section outlining the requirements for creating and managing these independent API instances. The changes are well-documented, providing clear rationale, examples, and cross-references to other parts of the specification. The addition addresses advanced use cases such as micro-frontends and dependency injection, enhancing the flexibility of the OpenFeature API.

[lukas-reining]

This looks good to me!