api: support get default config

[rleungx]

What problem does this PR solve?

TiDB dashboard needs to get the default config to compare with the existed config.

What is changed and how it works?

This PR adds API pd/api/v1/config/default to get the default config.

Check List

Tests

• Unit test

Release note

adds API pd/api/v1/config/default to get the default config.

[HunDunDM]

LGTM

[codecov-io]

Codecov Report

Merging #2258 into master will increase coverage by 0.04%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #2258      +/-   ##
==========================================
+ Coverage   76.53%   76.57%   +0.04%     
==========================================
  Files         199      199              
  Lines       21522    21529       +7     
==========================================
+ Hits        16472    16486      +14     
  Misses       3826     3826              
+ Partials     1224     1217       -7     

Impacted Files	Coverage Δ
server/api/config.go	39.22% <66.66%> (+0.72%)	⬆️
server/api/router.go	91.19% <100.00%> (+0.04%)	⬆️
server/region_syncer/server.go	81.25% <0.00%> (-10.94%)	⬇️
server/schedulers/shuffle_hot_region.go	63.96% <0.00%> (-9.91%)	⬇️
pkg/btree/btree.go	86.84% <0.00%> (-0.81%)	⬇️
server/config/option.go	90.04% <0.00%> (-0.44%)	⬇️
server/cluster/cluster.go	81.27% <0.00%> (+0.44%)	⬆️
client/base_client.go	91.03% <0.00%> (+0.68%)	⬆️
server/tso/tso.go	81.75% <0.00%> (+0.72%)	⬆️
server/member/lease.go	95.31% <0.00%> (+3.12%)	⬆️
... and 5 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 08802bf...717c447. Read the comment docs.

[nolouch]

/merge

[sre-bot]

/run-all-tests