add writeBatch

[tinect]

While such feature will come (thephpleague/flysystem#1778), I like to add a possibility by now.

To transfer many files will result in long-running tasks, e.g.:
Shopware assets:install: 2 hours and 37 minutes
With this change: 33 seconds

Shopware theme:compile: 30 secs
With this change: 5 secs

What do you think about it? Should I try adding tests?

[codecov]

Codecov Report

Attention: Patch coverage is 7.14286% with 39 lines in your changes missing coverage. Please review.

Project coverage is 73.43%. Comparing base (afb885b) to head (cd1a5a2).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/BunnyCDNClient.php	4.00%	24 Missing ⚠️
src/BunnyCDNAdapter.php	12.50%	14 Missing ⚠️
src/WriteBatchFile.php	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #81      +/-   ##
============================================
- Coverage     75.29%   73.43%   -1.86%     
- Complexity      122      129       +7     
============================================
  Files             3        4       +1     
  Lines           255      256       +1     
============================================
- Hits            192      188       -4     
- Misses           63       68       +5     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[tinect]

Hey @sifex
I hope you're doing well. I was wondering if you might have some time to have a look at the PR I've been working on? Your feedback would be greatly appreciated!

[sifex]

Thanks @tinect, Merged ☺️