[flagd] Intial version of in-process provider

[Kavindu-Dodan]

As mentioned in flagd documentation of Creating an in-process flagd provider ¹, in-process provider comes with unique advantages. This issue focuses on creating the first version of flagd in-process provider in Java.

Requirements

• Fulfill requirements outlined by flagd in-process guideline ¹
• Minimal external dependencies
• Updated/New ReadMe & usage guide
• Unit & Integration tests

Technical requirements

• Use the same repository directory structure (providers/flagd)
• Build both remote and in-process providers into a single dependency (To be decided after the POC)
• Reuse existing interfaces and implementations where possible

Tasks

• POC implementation and build to detect packaging requirements (no evaluations needed)
• In-process provider with gRPC flag sync capability on basic flags
• Support for JsonLogic evaluators (complex flagd flag configurations)
• Support for flagd custom JsonLogic evaluators

Footnotes

1. https://github.com/open-feature/flagd/blob/main/docs/other_resources/in-process-providers/specification.md ↩ ↩²