[Draft][Batch][Flex] Directly call async function in batch engine for flex mode.

[thy09]

Description

When the user provides an async function as the entry of flex flow, we are still using a process pool to run each lines.
However we can actually expose exec_line_async which directly call the async entry function, then we can leverage the async part of the batch engine.

All Promptflow Contribution checklist:

• The pull request does not introduce [breaking changes].
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.
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Testing Guidelines

• Pull request includes test coverage for the included changes.

[github-actions]

promptflow SDK CLI Azure E2E Test Result heta/call-async-function-directly

  4 files    4 suites   4m 14s ⏱️
201 tests 181 ✅ 20 💤 0 ❌
804 runs  724 ✅ 80 💤 0 ❌

Results for commit 9fd4c74.

[github-actions]

SDK CLI Test Result heta/call-async-function-directly

   12 files     12 suites   48m 29s ⏱️
  513 tests   494 ✅ 19 💤 0 ❌
2 052 runs  1 976 ✅ 76 💤 0 ❌

Results for commit 9fd4c74.

[github-actions]

Hi, thank you for your interest in helping to improve the prompt flow experience and for your contribution. We've noticed that there hasn't been recent engagement on this pull request. If this is still an active work stream, please let us know by pushing some changes or leaving a comment.

[github-actions]

Hi, thank you for your contribution. Since there has not been recent engagement, we are going to close this out. Feel free to reopen if you'd like to continue working on these changes. Please be sure to remove the no-recent-activity label; otherwise, this is likely to be closed again with the next cleanup pass.