[Release] Update change log

[D-W-]

Description

Please add an informative description that covers that changes made by the pull request and link all relevant issues.

Announce support for flex flow & prompty. This pull request introduces new features to the src/promptflow/CHANGELOG.md file. The two main additions are the introduction of 'flex flow' and 'prompty', both of which aim to streamline and simplify the development process.

New Features:

• src/promptflow/CHANGELOG.md: Added 'flex flow', a feature that allows for the design of powerful LLM apps using Python functions or classes. This feature also allows for seamless testing and running of logic with the VS Code Extension.
• src/promptflow/CHANGELOG.md: Introduced 'prompty', an experimental feature aimed at streamlining the creation of prompt templates. This feature simplifies development with .prompty files and enhances prompts with ease.

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.
• [ ] Create an issue and link to the pull request to get dedicated review from promptflow team. Learn more: suggested workflow.

General Guidelines and Best Practices

• [ ] Title of the pull request is clear and informative.
• [ ] There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• [ ] Pull request includes test coverage for the included changes.

[github-actions[bot]]

promptflow SDK CLI Azure E2E Test Result wanhan/update_readme_release

  4 files    4 suites   4m 11s :stopwatch: 242 tests 205 :white_check_mark:  37 :zzz: 0 :x: 968 runs  820 :white_check_mark: 148 :zzz: 0 :x:

Results for commit 2667f8fd.

:recycle: This comment has been updated with latest results.

[github-actions[bot]]

SDK CLI Global Config Test Result wanhan/update_readme_release

6 tests   6 :white_check_mark:  1m 11s :stopwatch: 1 suites  0 :zzz: 1 files    0 :x:

Results for commit 2667f8fd.

:recycle: This comment has been updated with latest results.

[github-actions[bot]]

Executor Unit Test Result wanhan/update_readme_release

796 tests   796 :white_check_mark:  3m 44s :stopwatch:   1 suites    0 :zzz:   1 files      0 :x:

Results for commit 2667f8fd.

:recycle: This comment has been updated with latest results.

[github-actions[bot]]

Executor E2E Test Result wanhan/update_readme_release

243 tests   238 :white_check_mark:  5m 36s :stopwatch:   1 suites    5 :zzz:   1 files      0 :x:

Results for commit 2667f8fd.

:recycle: This comment has been updated with latest results.

[github-actions[bot]]

SDK CLI Test Result wanhan/update_readme_release

    4 files      4 suites   1h 2m 37s :stopwatch:   715 tests   674 :white_check_mark:  41 :zzz: 0 :x: 2 860 runs  2 696 :white_check_mark: 164 :zzz: 0 :x:

Results for commit 2667f8fd.

:recycle: This comment has been updated with latest results.