refactor(abc:st): use Observable instead of Promise of load

[cipchk]

• close refactor(abc:st): use Observable instead of Promise of load ng-alain#2233

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ng-alain/delon/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[x] Yes
[ ] No

Other information

[codecov]

Codecov Report

Merging #1637 (c4ef806) into master (d815dbd) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

❗ Current head c4ef806 differs from pull request most recent head 98c0b67. Consider uploading reports for the commit 98c0b67 to get more accurate results

@@            Coverage Diff             @@
##           master    #1637      +/-   ##
==========================================
- Coverage   95.51%   95.48%   -0.03%     
==========================================
  Files         277      277              
  Lines        8561     8553       -8     
  Branches     1682     1683       +1     
==========================================
- Hits         8177     8167      -10     
- Misses        319      320       +1     
- Partials       65       66       +1     

Files Changed	Coverage Δ
packages/abc/st/st-data-source.ts	100.00% <100.00%> (ø)
packages/abc/st/st.component.ts	99.78% <100.00%> (-0.01%)	⬇️

... and 1 file with indirect coverage changes

[ng-alain-bot]

Preview is ready!