Add secrets manager integration for credential retrieval

[alberefe]

Integrate Perceval with external secrets managers (Bitwarden, HashiCorp Vault) so credentials can be resolved at runtime instead of being passed as plain-text CLI arguments.

• Add --secrets-manager, --item-name, and provider-specific CLI flags
  to BackendCommandArgumentParser
• Add _pre_init() and _build_manager() in BackendCommand to orchestrate credential resolution via grimoirelab-toolkit's resolve_credentials()
• Enable secrets manager support on 14 backends
• Add hvac as optional dependency group
• Usage in README

[alberefe]

I'll be fixing this later please wait before you check so every test and dependency is working fine!

[sduenas]

Please check my initial comments. Also, my concern is this only makes perceval work if you use the backend command but not the library. Please evaluate if it makes sense to integrate it within fetch or other methods, or if on the other hand, developers should use grimoirelab toolkit instead.

[sduenas]

Please fix the lint errors and add unit test for this functionality.

[alberefe]

Getting those errors cause I think it's importing an old version of Toolkit from before it had the las PR changes, is that correct? Not sure how to deal with that or if I'm even correct, but not sure what to do with it.

[sduenas]

Getting those errors cause I think it's importing an old version of Toolkit from before it had the las PR changes, is that correct? Not sure how to deal with that or if I'm even correct, but not sure what to do with it.

We fixed it in #900. Please rebase your branch.