Standardized Attribute Dictionary (Memorandum)

**What would you like to be added**:

We would like to propose a new **Memorandum GEP** establishing a standardized, vendor-neutral dictionary of protocol attributes anchored on [OpenTelemetry Semantic Conventions](https://opentelemetry.io/docs/specs/semconv/).

This dictionary will define canonical attribute keys, types, and associated semantics for use across Gateway API features.

**Why this is needed**:

As the Gateway API evolves, configurations increasingly require referencing dynamic attributes for connections, requests, responses, etc. Without a common specification, each implementation exposes its own native syntax, leading to vendor lock-in and broken portability. By defining a canonical dictionary anchored on stable OpenTelemetry Semantic Conventions, Gateway API decouples user-facing intent from the underlying implementation.

Start with the linked GEP overview and the OpenTelemetry Semantic Conventions referenced in the issue. The work is complete when the proposed Memorandum GEP establishes an agreed vendor-neutral dictionary of canonical attribute keys, types, and semantics for Gateway API features.

Written by the indexing model from the issue text.