feat: exposed factory interface to extend splunk_outputs

[dhruvanshus-crest]

Task - OTL-4482
splunk_ouptuts

• Updated splunkoutputsexporter.NewFactory(opts ...Option) to register the built-in httpout sub-exporter by default and apply external overrides at construction time.
• Added a public SubExporterFactory interface and WithSubExporter option so external repos can override the built-in [httpout] exporter creation.
• Added tests for httpout override behavior, ensuring only [httpout] is read, and preserving the existing error behavior when [httpout] is missing.
• Note: splunk_outputs currently supports overriding the existing [httpout] path only; generic output stanza schemes such as [tcpout] or [s2s] are not parsed.