feat: add default fees for mainnet/testnet networks

[maksymar]

This PR adds auto-setup for fees if not explicitly provided during initialisation.

Fees values hardcoded depending on config network (mainnet/testnet) and rely on docs.

[github-actions]

canbench 🏋 (dir: .) cd5eec5 2025-03-05 18:00:57 UTC

ℹ️ No significant performance changes detected 👍
✅ ./canbench_results.yml is up to date

~/work/bitcoin-canister/bitcoin-canister/scripts ~/work/bitcoin-canister/bitcoin-canister
~/work/bitcoin-canister/bitcoin-canister

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Benchmark: insert_300_blocks
  total:
    instructions: 475.59 M (no change)
    heap_increase: 10 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: get_metrics
  total:
    instructions: 7.63 M (no change)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: insert_block_headers
  total:
    instructions: 3.11 B (no change)
    heap_increase: 1 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: insert_block_headers_multiple_times
  total:
    instructions: 11.40 B (no change)
    heap_increase: 7 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: pre_upgrade_with_many_unstable_blocks
  total:
    instructions: 6.00 B (no change)
    heap_increase: 4097 pages (no change)
    stable_memory_increase: 1792 pages (no change)

  serialize_blocktree (scope):
    instructions: 2.40 B (no change)
    heap_increase: 2048 pages (no change)
    stable_memory_increase: 0 pages (no change)

  serialize_blocktree_flatten (scope):
    instructions: 183.74 K (no change)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

  serialize_blocktree_serialize_seq (scope):
    instructions: 2.40 B (no change)
    heap_increase: 2048 pages (no change)
    stable_memory_increase: 0 pages (no change)

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