Add atto namespace and CAIP-2 specification

[rotilho]

This PR proposes a new atto namespace for the Atto ecosystem and adds an initial CAIP-2 profile for Atto chain identifiers.

Why this should be its own namespace:

• Atto is a native layer-1 payment network, not a sub-environment of an existing namespace.
• It has a distinct account-chain model, native address format, native transaction format, and native proof-of-work validation rules.
• Cross-chain tools interacting with Atto need Atto-specific validation logic.