Encrypt logged HTTP secret header values.

[benashz]

When Terraform is executed with TF_LOG=debug all HTTP requests to Vault are logged in clear text. With the recently merged #775 it is possible that a long lived "non-batch" authentication token might be revealed. This PR adds support logging the HMAC'd header value for X-Vault-Token

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Relates #1040

Release note for CHANGELOG:

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'

...

[calvn]

I see that this is very similar to the RoundTrip code in the SDK but with the additional HMAC'ing logic, LGTM!

[benashz]

Request log lines now look like:

-----------------------------------------------------
2021/12/06 20:06:41 [DEBUG] Vault API Request Details:
---[ REQUEST ]---------------------------------------
GET /v1/auth/token/lookup-self HTTP/1.1
Host: localhost:8200
User-Agent: Go-http-client/1.1
X-Vault-Request: true
X-Vault-Token: hmac-sha256:5c02513df6132c3ad5e40008b38ee5e2f56946e4e2119f0ecbbf93d4048ba3eb
Accept-Encoding: gzip