[FEATURE] Specify and clarify configuration parsing for flag configurations

Requirements

How we are parsing and handling flags is different from implementation to implementation. Simply based on the fact how we grew as a project and that a lot of implementations have been done by multiple contributors. Now it is the time to rethink our parsing and handling of the configurations, especially for in-process.

Goals

• utilize schema validation as much as possible, instead of custom logic, on a flag and not on a configuration level
• define which file formats are supported (relates too https://github.com/open-feature/flagd/issues/1689)
• define what can cause a PARSE_ERROR: currently only an invalid targeting rule can cause this error, other schema invalidation result in a FLAG_NOT_FOUND
• define this in a way that we can test this via our testbed/test-harness

issue relating to this

• https://github.com/open-feature/flagd/issues/1627
• https://github.com/open-feature/flagd/issues/1689
• https://github.com/open-feature/flagd/issues/1487
• https://github.com/open-feature/flagd/issues/1679

Start by reviewing the linked issues 1627, 1689, 1487, and 1679 to understand the existing parsing decisions and unresolved requirements. Use the testbed/test-harness to define expected behavior for schema validation, supported formats, and PARSE_ERROR versus FLAG_NOT_FOUND outcomes; the work is done when those rules are specified and testable.