feat: add configurable RPC concurrency via RPC_CONCURRENCY env var

[fakedev9999]

Summary

• Add RPC_CONCURRENCY environment variable to configure concurrent RPC requests
• Default value of 10 preserves existing behavior (no regression)
• Users hitting 429 rate limits can set lower values (e.g., 3-5)

Problem

Users with low-tier RPC plans experience 429 rate limit errors due to hardcoded .buffered(10) and .buffered(15) values throughout the codebase.

Solution

Make RPC concurrency configurable via RPC_CONCURRENCY environment variable while preserving the default behavior for existing users.

Changes

• utils/host/src/fetcher.rs: Add DEFAULT_RPC_CONCURRENCY constant, get_rpc_concurrency_from_env() helper, and concurrency field to RPCConfig
• utils/host/src/block_range.rs: Use configurable concurrency
• validity/src/proposer.rs: Use fetcher's concurrency config
• scripts/utils/bin/cost_estimator.rs: Use configurable concurrency (2 locations)
• scripts/utils/bin/gen_sp1_test_artifacts.rs: Use configurable concurrency
• book/: Document RPC_CONCURRENCY in proposer docs and add troubleshooting section

Usage

# For low-tier RPC plans (free tiers)
export RPC_CONCURRENCY=3

# For basic paid tiers
export RPC_CONCURRENCY=5

# Default (professional tiers) - no change needed
# RPC_CONCURRENCY=10

TODO:
- Manual test/benchmark with low RPC_CONCURRENCY value
- Verify no performance regression with default value