Pre-generate test blobs bundle to improve test time. (#4829)

## Issue Addressed

Addresses #4778, and potentially fixes the flaky deneb builder test `builder_works_post_deneb`.

The [deneb builder test](c5c84f1213/beacon_node/http_api/tests/tests.rs (L5371)) has been quite flaky on our CI (`release-tests`) since it was introduced. I'm guessing that it might be timing out on the builder `get_header` call (1 second), and therefore the local payload is used, while the test expects builder payload to be used. 

On my machine the [`get_header` ](c5c84f1213/beacon_node/execution_layer/src/test_utils/mock_builder.rs (L367)) call takes about 550ms, which could easily go over 1s on slower environments (our windows CI runner is much slower than the ubuntu one).

I did a profile on the test and it showed that `blob_to_kzg_commiment` and `compute_kzg_proof` was taking a large chunk of time, so perhaps pre-generating the blobs could help stablise this test.

## Proposed Changes

Pre-generate blobs bundle for Mainnet and Minimal presets.

Before the change `get_header` took about **550ms**, and it's now reduced to **50-55ms** after the change. If timeout was indeed the cause of the flaky test, this fix should stablise it. This also brings the flaky `builder_works_post_deneb` test time from 50s to 10s. (8s if we only use a single blob)

beacon_node/execution_layer/src/test_utils/mod.rs

@@ -25,8 +25,8 @@ use warp::{http::StatusCode, Filter, Rejection};
 
 use crate::EngineCapabilities;
 pub use execution_block_generator::{
-    generate_genesis_block, generate_genesis_header, generate_pow_block, generate_random_blobs,
-    Block, ExecutionBlockGenerator,
+    generate_blobs, generate_genesis_block, generate_genesis_header, generate_pow_block, Block,
+    ExecutionBlockGenerator,
 };
 pub use hook::Hook;
 pub use mock_builder::{MockBuilder, Operation};