Enhancement: Allow http headers to be marked as sensitive

[batesandy]

Hi there,

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Terraform Version

0.12.x

Affected Resource(s)

Please list the resources as a list, for example:
http

Enhancement detail

headers may contain sensitive information such as persistent auth information.
Woud be useful to be able to mark as sensitive so that the header is not echoed to any plan/apply output
Treated in same way as sensitive values in state files as other providers