out_azure_blob: add `log_key` option

**Is your feature request related to a problem? Please describe.**
This feature is implemented in this PR: https://github.com/fluent/fluent-bit/pull/9791
However, the implementation has bugs in it. The output only transports 1 line per file. It will ignore all other lines in the file.

**Describe the solution you'd like**
As a solution, could we fix the code in the PR

Start with the out_azure_blob implementation in pull request #9791 and reproduce its handling of a file containing multiple lines. Trace why only one line is transported, then verify that all lines are sent while preserving the requested log_key behavior.

Written by the indexing model from the issue text.