feat: Support single-canister project definitions

[rikonor]

Summary:

This PR enhances the project manifest (icp.yaml) to support a simplified configuration for single-canister projects. Previously, only a canisters list was supported. Now, a project with a single canister can be defined directly using a top-level canister key, improving developer experience for simpler projects. The existing canisters key for multi-canister projects remains fully supported.

Detailed Changes:

• Flexible Project Manifest (lib/icp-project/src/model.rs):

  • The RawProjectManifest struct was modified to use #[serde(flatten)] on an Option<RawCanistersField>. This allows serde to deserialize from either a canister key (for a single canister) or a canisters key (for multiple canisters) at the top level of icp.yaml.
  • The ProjectManifest::load function was updated to handle this Option. If neither canister nor canisters is present in the manifest, it now correctly falls back to the default behavior of searching for canisters with the canisters/* glob pattern.

• Unified Build Logic (bin/icp-cli/src/commands/build.rs):

  • The build command's execution logic was updated to normalize the canister configuration. It now uses a match statement to handle both the single-canister (CanistersField::Canister) and multi-canister (CanistersField::Canisters) variants, creating a unified list of canisters to be built.
  • This ensures that the downstream build process can iterate over a consistent data structure, regardless of how the canisters are defined in the manifest.

Configuration Examples:

This change makes both of the following icp.yaml formats valid:

Single-Canister Project (New):

canister:
  name: my-canister
  build:
    adapter:
      type: script
      command: echo "building my canister"

Multi-Canister Project (Existing):

canisters:
  - canisters/*

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