[EPM] Handle constant_keyword type in KB index patterns and ES index templates

[skh]

Summary

Implements #61112

For index templates, adds a unit test to verify the behavior as the type was already handled by default.
For index patterns, constant_keyword is treated like a keyword, i.e. as an aggregatable string.

To test this:

• Package installation should work as before.
• Check the newly added unit tests if the behavior is correct.

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[ruflin]

Check if the created templates contain the correct fields and they do:

              "stream" : {
                "properties" : {
                  "namespace" : {
                    "type" : "constant_keyword"
                  },
                  "type" : {
                    "type" : "constant_keyword"
                  },
                  "dataset" : {
                    "type" : "constant_keyword"
                  }
                }

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: d4822a8

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream