feat: add a ping() API to check status of OSS and Cloud instance

[bednar]

Proposed Changes

Added a ping() API to check status of OSS and Cloud instance.

Checklist

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• make test completes successfully
• Commit messages are in semantic format
• Sign CLA (if not already signed)

[codecov-commenter]

Codecov Report

Merging #97 (6e5ef37) into master (834fc2d) will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master      #97      +/-   ##
============================================
+ Coverage     81.88%   82.07%   +0.19%     
- Complexity      379      380       +1     
============================================
  Files            23       23              
  Lines           900      904       +4     
============================================
+ Hits            737      742       +5     
+ Misses          163      162       -1     

Impacted Files	Coverage Δ
src/InfluxDB2/Client.php	94.44% <100.00%> (+0.69%)	⬆️
src/InfluxDB2/HeaderSelector.php	73.91% <0.00%> (+4.34%)	⬆️

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