feat: all backends support making them as read-only

[AJIOB]

For the local developers we want to provide the access to the remote cache server in read-only mode.

So, this PR allows read-only access to read-write storages via per-storage specific flag

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.54217% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.56%. Comparing base (5d52f91) to head (5e18212).

Files with missing lines	Patch %	Lines
src/config.rs	88.69%	13 Missing ⚠️
src/cache/cache.rs	78.43%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2705      +/-   ##
==========================================
+ Coverage   74.51%   74.56%   +0.04%     
==========================================
  Files          70       70              
  Lines       39652    39793     +141     
==========================================
+ Hits        29546    29670     +124     
- Misses      10106    10123      +17     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[AJIOB]

Hi @sylvestre . Is it ok to you?

[sylvestre]

please add more tests:

• No test for the check() short-circuit returning ReadOnly when rw_mode is set
• No test for cache_mode_from_env_var parsing

[AJIOB]

Thanks, will fix the PR.

Looks like the read-only short-circuit was tested via test_operator_storage_redis_with_read_only (was added in this PR)

[AJIOB]

I've added 2 tests in that PR:

• test_operator_storage_redis_with_read_only to check read-only short-curcuit via remote storage access.. The local storage logic wasn't changed, so not tested
• test_cache_mode_from_env_var to check the logic of new function cache_mode_from_env_var

[AJIOB]

@sylvestre , I've fixed all you suggested, thanks. Is it OK now?

[AJIOB]

Hi @sylvestre,

Graceful ping about the PR