docs: Add proposal for Canton DeFi composability standards

This pull request adds a comprehensive development fund proposal for establishing core DeFi composability standards on the Canton platform. The proposal outlines the design, specification, and reference implementation of two new Canton Improvement Proposals (CIPs): a tokenized vault standard and a set of DeFi composability interfaces. The document details objectives, technical specifications, milestones, funding, and ecosystem impact.

Key additions and changes include:

New DeFi Standards and Interfaces

• Introduces CIP-TBD-A (Vault Standard) and CIP-TBD-B (DeFi Composability Interfaces), specifying canonical Daml interfaces for vaults (Vault, VaultShare), pricing, swaps, and oracles, modeled after proven EVM standards like ERC-4626 and Uniswap V2.
• Provides detailed interface design decisions, including lifecycle methods, accounting, rounding, privacy, and composability patterns, ensuring compatibility with existing daml-finance primitives and CIP-112.

Reference Implementations and Ecosystem Alignment

• Outlines reference implementations for each interface: SimpleVault, SimplePricing, SimpleSwap, and PushOracle, with integration and composability tests to demonstrate cross-protocol compatibility.
• Ensures backward compatibility with existing standards (CIP-56, CIP-112) and describes how the new interfaces can be incrementally adopted by current Canton DeFi projects. (Fb