Doc: Update headers to indicate level of support for plugins

[karenzone]

Related: #15970

For POC, we're starting with these documented support levels:

• Elastic Tier 1 (t1)
• Elastic Tier 2 (t2)
• Community plugin (comm)
• Partner plugin (partner)

This PR adds the support level to plugin header files for the Logstash Reference (LSR):

• plugin_header.asciidoc
• plugin_header-integration.asciidoc

Notes

• "Advertised" and core plugins don't use a header file. Custom information is included in the source file.
• Decisions on how we'll support attributes are ongoing, and may impact this implementation.

Next steps

• If this change gets picked up, it will need to be replicated for the Versioned Plugin Reference (VPR) header files in the logstash-docs repo:
  • plugin_header-integration.asciidoc
  • plugin_header.asciidoc

[karenzone]

Update

Closing without merging pending decisioning.
More info in #15970.