Empty auth-scheme support

What did you do?
I want to scrape target secured via Authorization header, curl example:

curl -H "Authorization: $token" ...

Per docs i should be able to not use Bearer
https://prometheus.io/docs/prometheus/latest/configuration/configuration/#scrape_config

authorization:
  # Sets the authentication type of the request.
  [ type: <string> | default: Bearer ]

So i've set:

authorization:
  type: ''

But in tcpdump i still see that Bearer prefix is added before token.
Validation code:
https://github.com/prometheus/common/blob/f57586dab602b7ae2607c552438a23a551f38773/config/http_config.go#L232

What did you expect to see?
I expect to being able to choose any auth-scheme, including nil one.

What did you see instead? Under which circumstances?
I see that Bearer auth-scheme is forced, i.e.:

curl -H "Authorization: Bearer $token" ...

instead of:

curl -H "Authorization: $token" ...

Environment

• Prometheus version:

prometheus, version 2.31.1 (branch: HEAD, revision: 411021ada9ab41095923b8d2df9365b632fd40c3)
  build user:       root@9419c9c2d4e0
  build date:       20211105-20:35:02
  go version:       go1.17.3
  platform:         linux/amd64

• Prometheus configuration file:

scrape_configs:
  - job_name: "auth"
    authorization:
      type: ''
      credentials: SECRET_TOKEN
    static_configs:
    - targets: 
      - a.b.c:443

This could be fixed by verifying that type is explicitly set to empty string before callingTrimSpace. I can send PR if such a solution is fine.

Start in config/http_config.go at the validation code linked in the issue, then reproduce the scrape configuration with an explicitly empty authorization type and credentials. Verify the generated Authorization header against the expected curl form, ensuring an empty scheme does not add the default Bearer prefix.