Support token passed in Authorization Bearer HTTP header

[uepoch]

Is your feature request related to a problem? Please describe.
Some software (e.g prometheus: prometheus/prometheus#2346, prometheus/prometheus#1724) don't support passing arbitrary headers, and only permit users to configure Authorization headers

Describe the solution you'd like
Consider Authorization: Bearer <token> as a valid mechanism to pass a Vault token

Explain any additional use-cases
Some old-world sdk's are only configured to handle Authorization as a secret passed in Bearer (JWK, simple secret-id, ... ) and this could ease integration with mid-level http client lib in some cases.

Additional context
Already has been looked into in consul via hashicorp/consul#4483