[receiver/splunkhec] Support passing access token via context

[atoulme]

Component(s)

receiver/splunkhec

Is your feature request related to a problem? Please describe.

The Splunk HEC receiver currently sets an access token passthrough value as a resource attribute on the payload.

Describe the solution you'd like

Instead, the receiver should set this value in the context of the request so it may be passed around to exporters without exposing a secret in the data.

Describe alternatives you've considered

No response

Additional context

No response

[github-actions]

Pinging code owners:

• receiver/splunkhec: @atoulme

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