config: make original HTTP API compatible with config manager

[rleungx]

What problem does this PR solve?

Part of #1977. It should be reviewed after #2049 is merged.

What is changed and how it works?

This PR makes the original config API support config manager.

Check List

Tests

• Unit test

[codecov-io]

Codecov Report

Merging #2080 into master will decrease coverage by 0.03%.
The diff coverage is 81.35%.

@@            Coverage Diff             @@
##           master    #2080      +/-   ##
==========================================
- Coverage    76.7%   76.67%   -0.04%     
==========================================
  Files         194      194              
  Lines       20003    20036      +33     
==========================================
+ Hits        15344    15363      +19     
- Misses       3503     3511       +8     
- Partials     1156     1162       +6

Impacted Files	Coverage Δ
server/schedulers/balance_leader.go	91.33% <100%> (+0.48%)	⬆️
server/schedulers/balance_region.go	85.15% <100%> (+0.6%)	⬆️
server/schedule/placement/rule.go	94.28% <100%> (+0.34%)	⬆️
pkg/mock/mockcluster/mockcluster.go	90.19% <100%> (+0.29%)	⬆️
server/schedule/placement/rule_manager.go	86.45% <50%> (-2.44%)	⬇️
server/core/storage.go	72.86% <73.68%> (+0.46%)	⬆️
server/schedulers/shuffle_region.go	72.85% <0%> (-11.43%)	⬇️
server/schedulers/base_scheduler.go	60.86% <0%> (-8.7%)	⬇️
server/tso/tso.go	77.37% <0%> (-2.92%)	⬇️
server/region_syncer/client.go	79.31% <0%> (-1.73%)	⬇️
... and 11 more

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[rleungx]

/run-all-tests

[rleungx]

/run-all-tests

[overvenus]

Rest LGTM

[overvenus]

How about returning an error? I think we should be strict about unknown types in case of unexpected errors.

[rleungx]

/run-all-tests

[overvenus]

LGTM

[sre-bot]

/run-all-tests