[Filebeat] CEL input - set the default User-Agent

[andrewkroh]

Describe the enhancement:

Set a User-Agent string for the CEL input by default that identifies it as Filebeat and includes the version.

Describe a specific use case for the enhancement or feature:

When monitoring the logs of services that Fleet integrations connect to or proxies the requests pass through, it is very useful to have an identifiable User-Agent string. The current header from CEL defaults to:

User-Agent: Go-http-client/1.1

[elasticmachine]

Pinging @elastic/security-service-integrations (Team:Security-Service Integrations)

[efd6]

The design intention here was to do that in the CEL program since different programs really should have different UAs.