Spark/Lightning: add reader_worker_count and reader_pool_type

[chongxiaoc]

Checklist before submitting

• Did you read the contributor guide?
• Did you update the docs?
• Did you write any tests to validate this change?
• Did you update the CHANGELOG, if this change affects users?

Description

Fixes # (issue).

Review process to land

1. All tests and other checks must succeed.
2. At least one member of the technical steering committee must review and approve.
3. If any member of the technical steering committee requests changes, they must be addressed.

[github-actions]

Unit Test Results

     893 files  +   405       893 suites  +405   10h 3m 7s ⏱️ + 2h 56m 57s
     781 tests ±       0       737 ✔️ +   106       44 💤  -    106  0 ❌ ±0 
19 061 runs  +8 479  13 666 ✔️ +6 314  5 395 💤 +2 165  0 ❌ ±0 

Results for commit 102ee5a. ± Comparison against base commit ba98ad5.

[github-actions]

Unit Test Results (with flaky tests)

  1 035 files  +     519    1 035 suites  +519   10h 44m 3s ⏱️ + 3h 33m 52s
     781 tests ±         0       737 ✔️ +   106       44 💤  -    106  0 ❌ ±0 
22 367 runs  +11 237  15 594 ✔️ +7 952  6 773 💤 +3 285  0 ❌ ±0 

Results for commit 102ee5a. ± Comparison against base commit ba98ad5.

[Tixxx]

LGTM