[Improvement](executor)add remote scan thread pool

[wangbo]

Proposed changes

add remote scan thread pool for workload group.

[doris-robot]

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[wangbo]

run buildall

[doris-robot]

TeamCity be ut coverage result: Function Coverage: 35.70% (8548/23943) Line Coverage: 27.52% (69365/252021) Region Coverage: 26.68% (35981/134868) Branch Coverage: 23.49% (18391/78306) Coverage Report: http://coverage.selectdb-in.cc/coverage/c8079ae76d38108645e7d2dca784e86152d1ecd4_c8079ae76d38108645e7d2dca784e86152d1ecd4/report/index.html

[wangbo]

run buildall

[wangbo]

run buildall

[doris-robot]

TeamCity be ut coverage result: Function Coverage: 35.69% (8546/23943) Line Coverage: 27.51% (69341/252021) Region Coverage: 26.66% (35961/134868) Branch Coverage: 23.48% (18384/78306) Coverage Report: http://coverage.selectdb-in.cc/coverage/fa183583b0dbade9cbcd37b3d8b0b4e501a15dd1_fa183583b0dbade9cbcd37b3d8b0b4e501a15dd1/report/index.html

[github-actions[bot]]

PR approved by at least one committer and no changes requested.

[github-actions[bot]]

PR approved by anyone and no changes requested.

[wangbo]

run buildall

[doris-robot]

TeamCity be ut coverage result: Function Coverage: 35.72% (8548/23929) Line Coverage: 27.53% (69362/251973) Region Coverage: 26.67% (35970/134855) Branch Coverage: 23.48% (18385/78304) Coverage Report: http://coverage.selectdb-in.cc/coverage/17e6bfc64aa051fdecbd202abb52a551dc52f780_17e6bfc64aa051fdecbd202abb52a551dc52f780/report/index.html

[github-actions[bot]]

PR approved by at least one committer and no changes requested.