feat: add Discovery Extension for API versioning and capabilities

[phessophissy]

Description

This PR adds a new Discovery Extension (Section 1.4.2.12) that enables API consumers to discover the version of the L2 Block Explorer API Standard being implemented and which extensions are available.

Motivation

This directly addresses the contribution suggestion in Section 3.1:

"Versioning and Available Extensions: The presented standard is designed to be modular and evolving. However, the API endpoints currently do not include any information where the user could learn about the version of the standard that the API implements. It also does not include an endpoint for learning which extensions are implemented and can be queried."

New Endpoints

Endpoint	Description
GET /api/info	Returns API version, chain info, and implementation details
GET /api/extensions	Returns list of supported extensions with their status

New Requirements

ID	Description	Type
X_DIS_1	Endpoint to retrieve API standard version	Core
X_DIS_2	Endpoint to retrieve L2 chain information	Core
X_DIS_3	Endpoint to list supported extensions	Core
X_DIS_4	Version information for each extension	Optional
X_DIS_5	L1 chain information	Optional

Files Changed

• README.md - Added Section 1.4.2.12 and updated API spec list
• open-api/1_4_2_12_discovery.yml - New OpenAPI specification

Checklist

• Follows existing documentation style
• OpenAPI spec follows existing conventions
• Requirements follow ID naming pattern (X_DIS_*)
• Added to API Specification list in Section 1.3