Balance fetching for Litecoin and Decred

[oritwoen]

`balance::fetch()` only supports Bitcoin (mempool.space) and Ethereum (Etherscan). All other chains return `UnsupportedChain`.

The project already has puzzles on Litecoin and Decred (zden collection), and `Chain` already has explorer URLs for both:

• Litecoin: `blockchair.com/litecoin/transaction/{txid}`
• Decred: `dcrdata.decred.org/tx/{txid}`

Both blockchains have public APIs that work similarly to mempool.space:

• Litecoin: Blockchair REST API or a mempool.space-compatible instance (litecoinspace.org uses the same API format)
• Decred: dcrdata has a REST API at `dcrdata.decred.org/api/address/{addr}`

Running `boha balance zden/litecoin_segwit` currently fails with "Unsupported chain: Litecoin".