plugin_proxy: enable event_type specification for proxy plugins (4.2 Backport)

[jmccormick7]

This PR adds event_type to the flb_plugin_proxy_def struct. This allows for the setting of event_type to a value other than the default log only initialization.

This PR is a backport for FluentBit 4.2 that matches the approved changes in PR #11011

In the flb_plugin_proxy_register function default behavior is enforced by checking for unset event_type values. When unset (a value of 0) then log behavior is defaulted. No need for incorrect value checking since this was not a set field in the past. Unset fields are set as current behavior dictates and future use cases using incorrect values can be treated as user error.

Input use case does not use event-type and is unaffected by this change.

Addresses https://github.com/fluent/fluent-bit/issues/10995

This corresponding PR in the fluent-bit-go library enables the ability to allow Go plugins to register their event types https://github.com/fluent/fluent-bit-go/pull/80

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[jmccormick7]

@cosmo0920 I added this and one other pr (back porting to 4.1). I saw that the main pr was added to the 5.0 milestone. I just wanted to ask about the possibility of getting the back port in an earlier release for 4.2 as this is backwards compatible. I completely understand if it's preferred to wait until a major release as it is a feature change. I just figured it was worth checking in on to see if it could get released sooner. I'm more than happy to do anything additional that may be needed to support this if there's anything missing or areas where this falls short. Thanks!