keyspace: set keyspace name by env

[ystaticy]

What problem does this PR solve?

Issue Number: ref #53296

Problem Summary:

What changed and how does it work?

Check List

Tests

• Unit test
• Manual test (add detailed scripts or steps below)

1. Start PD and TiKV servers.
2. set env:

export KEYSPACE_NAME="ks1"

3. start TiDB server:
   We can see log:

["using API V2."] [keyspaceName=ks1]

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

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[tiprow]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.5473%. Comparing base (f0af776) to head (b7aa8e8).
Report is 8 commits behind head on master.

Additional details and impacted files

@@               Coverage Diff                @@
##             master     #53294        +/-   ##
================================================
+ Coverage   74.3523%   74.5473%   +0.1949%     
================================================
  Files          1494       1494                
  Lines        356302     429822     +73520     
================================================
+ Hits         264919     320421     +55502     
- Misses        72035      89468     +17433     
- Partials      19348      19933       +585     

Flag	Coverage Δ
integration	49.0803% <62.5000%> (?)
unit	71.2612% <100.0000%> (-1.8450%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
dumpling	53.9957% <ø> (-2.3014%)	⬇️
parser	∅ <ø> (∅)
br	50.4484% <ø> (+7.4815%)	⬆️