Add Bulletin chainspecs for Polkadot, Westend, and Paseo

[x3c41a]

Summary

• Add chainspecs/ directory with Bulletin chain specifications copied from paritytech/chainspecs
• Covers Polkadot, Westend, and Paseo (next + next v2)
• Flat layout, one file per network

Structure

chainspecs/
├── polkadot-chainspec.json
├── westend-chainspec.json
├── paseo-next-chainspec.json
└── paseo-next-v2-chainspec.json

Enables

• Merging of Add polkadot-bulletin-chain as submodule chainspecs#1263

Closes

• Supersedes Add Bulletin chainspecs for Polkadot, Westend, and Paseo #523
• Partially addresses [Cleanup] Have all bulletin code base in a single repository as much as possible #499

[x3c41a]

pushing changes on top of #523 instead.