feat: Add stub for Bitcoin headers endpoint

[dragoljub-duric]

As stated in here, candid interface should look as:

type bitcoin_block_header = blob;

type get_block_headers_request = record {
	start_height : nat32;
	end_height: opt nat32;
};

type get_block_headers_result = record {
	tip_height: nat32;
	block_headers: vec bitcoin_block_header;
};

bitcoin_get_block_headers : (get_block_headers_request) -> (get_block_headers_response);

This PR will add no-op endpoint for returning the block headers in the follow-up we will implement specified function.

[github-actions]

canbench 🏋 (dir: .)

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

---------------------------------------------------

Benchmark: insert_300_blocks
  total:
    instructions: 561.77 M (-0.00%) (change within noise threshold)
    heap_increase: 6 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: get_metrics
  total:
    instructions: 87.00 M (-0.00%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: insert_block_headers
  total:
    instructions: 3.90 B (-0.00%) (change within noise threshold)
    heap_increase: 2 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

Benchmark: insert_block_headers_multiple_times
  total:
    instructions: 13.90 B (-0.00%) (change within noise threshold)
    heap_increase: 7 pages (no change)
    stable_memory_increase: 0 pages (no change)

---------------------------------------------------

[ielashi]

Looks good! Some minor nitpicks. Also, I'd avoid linking to private documentation in the PR description as external readers can't access them. You can instead link to the public documentation: dfinity/interface-spec#298