Add Tag parameter to fluentbit syslog input

[LeonLow97]

What this PR does / why we need it:

Add tag field to Fluentbit syslog input so users can assign tags to syslog messages at the input level to route logs to specific parsers.

Which issue(s) this PR fixes:

Fixes #1732

Does this PR introduced a user-facing change?

Add `tag` field to the FluentBit Syslog ClusterInput. 
Assign a syslog input tag to CRD on `.spec.syslog.tag` 
so the FluentBit operator can route logs to different parsers.

Added a Unit Test for adding this tag (it might not be the most meaningful test but it's good to have).

Ran `make manifests` to update the manifests.

Additional documentation, usage docs, etc.:

Updated /input/syslog.md to document the new `tag` field.

[LeonLow97]

Hi @cw-Guo , this pull request is ready for review, could you help to take a look at it? Thank you! ☺️