cluster: check tiflash status by requesting it's api

[AstroProfundis]

What problem does this PR solve?

Close #1591

What is changed and how it works?

Implement a custom Ready() method for TiFlash instances.

Check List

Tests

• Manual test (add detailed scripts or steps below)

Code changes

• Has exported function/method change
• Has interface methods change

Release notes:

NONE

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• nexustar

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[codecov-commenter]

Codecov Report

Merging #1600 (3e2af72) into master (e4b380e) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

❗ Current head 3e2af72 differs from pull request most recent head 5761491. Consider uploading reports for the commit 5761491 to get more accurate results

@@            Coverage Diff             @@
##           master    #1600      +/-   ##
==========================================
- Coverage   15.74%   15.71%   -0.04%     
==========================================
  Files         149      149              
  Lines       15061    15098      +37     
==========================================
  Hits         2372     2372              
- Misses      12171    12208      +37     
  Partials      518      518              

Flag	Coverage Δ
integrate	15.71% <0.00%> (-0.04%)	⬇️
tiup	15.71% <0.00%> (-0.04%)	⬇️
unittest	?

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

Impacted Files	Coverage Δ
pkg/cluster/spec/instance.go	0.00% <0.00%> (ø)
pkg/cluster/spec/tiflash.go	0.00% <0.00%> (ø)

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[AstroProfundis]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 5761491