Support a list of observables for expect

[ytdHuang]

Checklist

Thank you for contributing to QuantumToolbox.jl! Please make sure you have finished the following tasks before opening the PR.

• Please read Contributing to QuantumToolbox.jl.
• Any code changes were done in a way that does not break public API.
• Appropriate tests were added and tested locally by running: make test.
• Any code changes should be julia formatted by running: make format.
• All documents (in docs/ folder) related to code changes were updated and able to build locally by running: make docs.
• (If necessary) the CHANGELOG.md should be updated (regarding to the code changes) and built by running: make changelog.

Request for a review after you have completed all the tasks. If you have not finished them all, you can also open a Draft Pull Request to let the others know this on-going work.

Description

As title

Related issues or PRs

close #374

[codecov]

Codecov Report

Attention: Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.

Project coverage is 93.32%. Comparing base (9cc0224) to head (ca79557).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/qobj/functions.jl	95.83%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #376      +/-   ##
==========================================
+ Coverage   93.18%   93.32%   +0.13%     
==========================================
  Files          43       43              
  Lines        2790     2803      +13     
==========================================
+ Hits         2600     2616      +16     
+ Misses        190      187       -3     

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