Make framework.WALPrefix a local path

[Zlaticanin]

The WAL rollback process for plugins is a way to have plugins cleanup external state from any failed requests. The intention is that, whenever a request comes in, the plugin is responsible for writing to framework.PutWAL() with some data, and then end a request by calling framework.DeleteWAL() to delete the data. If the request fails partway through, then DeleteWAL never gets called.
This PR makes framework.WALPrefix to be a local path so that performance secondaries wouldn't end up with the WAL data getting replicated.

Before fix:

2023-10-13T16:41:33.979-0700 [DEBUG] perf-pri.core0.auth.local-azure.auth_local-azure_b0e813b6.local-azure.vault-plugin-auth-azure: rolling back config: timestamp=2023-10-13T16:41:33.978-0700
2023-10-13T16:41:37.676-0700 [DEBUG] perf-sec.core0.auth.local-azure.auth_local-azure_b0e813b6.local-azure.vault-plugin-auth-azure: rolling back config: timestamp=2023-10-13T16:41:37.676-0700

After fix:

2023-10-13T16:49:20.192-0700 [DEBUG] perf-pri.core0.auth.local-azure.auth_local-azure_da705d54.local-azure.vault-plugin-auth-azure: rolling back config: timestamp=2023-10-13T16:49:20.191-0700

Azure Secrets PR: hashicorp/vault-plugin-secrets-azure#164

[fairclothjm]

Thanks! Can we please add a Changelog entry?

Are we also going to make the same change for Azure secrets?

Edit: I didn't see the link to the azure secrets PR. Thanks!