tests(parameters): add comments to SecretsProvider e2e test

[am29d]

Description of your changes

I have added a top comment in the e2e test file describing all the cases with setup and verification steps. I have also added a short guidance for future maintainers and contributors when adding new tests. Some of these pointers are generic, i.e. use dedicated resources for each test case. Others are more specific, i.e. we use custom function from CustomFile.ts to do X.

How to verify this change

Related issues, RFCs

Issue number: #1279

PR status

Is this ready for review?: YES
Is it a breaking change?: NO

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding changes to the examples
• My changes generate no new warnings
• The code coverage hasn't decreased
• I have added tests that prove my change is effective and works
• New and existing unit tests pass locally and in Github Actions
• Any dependent changes have been merged and published
• The PR title follows the conventional commit semantics

Breaking change checklist

• I have documented the migration process
• I have added, implemented necessary warnings (if it can live side by side)

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[dreamorosi]

Thank you for the PR, just left one minor suggestion to fix a typo.

Other than that, it looks great!