chore: Move token definition into a secret (#192)

<!-- markdownlint-disable-next-line first-line-heading -->
## Description

Moves the userinfo auth token field into a secret environment variable.
This means that we won't accidentally save api key values into git
(because secret values aren't saved to file)

## Context

<!-- Why is this change required? What problem does it solve? -->

## Type of changes

<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply. -->

- [x] Refactoring (non-breaking change)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would change existing
functionality)
- [ ] Bug fix (non-breaking change which fixes an issue)

## Checklist

<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->

- [x] I have followed the code style of the project
- [ ] I have added tests to cover my changes
- [ ] I have updated the documentation accordingly
- [ ] This PR is a result of pair or mob programming
- [ ] Exceptions/Exclusions to coding standards (e.g. #noqa or #NOSONAR)
are included within this Pull Request.

---

## Sensitive Information Declaration

To ensure the utmost confidentiality and protect your and others
privacy, we kindly ask you to NOT including [PII (Personal Identifiable
Information) / PID (Personal Identifiable
Data)](https://digital.nhs.uk/data-and-information/keeping-data-safe-and-benefitting-the-public)
or any other sensitive data in this PR (Pull Request) and the codebase
changes. We will remove any PR that do contain any sensitive
information. We really appreciate your cooperation in this matter.

- [ ] I confirm that neither PII/PID nor sensitive data are included in
this PR and the codebase changes.

Author: ian-robinson-35

bruno/gateway-api/collections/APIM/User_Info.yml

@@ -5,13 +5,15 @@ info:
 
 http:
   method: GET
-  url: https://internal-dev.api.service.nhs.uk/oauth2-mock/userinfo
+  url: https://{{host}}/oauth2-mock/userinfo
   auth:
     type: bearer
-    token: ""
+    token: "{{token}}"
 
 settings:
   encodeUrl: true
   timeout: 0
   followRedirects: true
   maxRedirects: 5
+
+docs: Use the `token` environment variable to set the Authorization header. Do not enter it directly into the header. This ensures that the value is never saved into git.

bruno/gateway-api/collections/APIM/environments/internal-dev.yml

@@ -0,0 +1,6 @@
+name: internal-dev
+variables:
+  - name: host
+    value: internal-dev.api.service.nhs.uk
+  - secret: true
+    name: token