[Security] SECVULN-8621: Fix XSS Vulnerability where content-type header wasn't explicitly set in API requests

[NiniOak]

Description

• Modified existing ensureContentTypeHeader middleware to set the response Content-Type to match the type of content that the server will process.

SOLUTION

ensureContentTypeHeader middleware is used to determine if the content-type set in the API request is the expected content type. If it's not a match:

• A warning is logged in Consul
• The appropriate contentType Header set before processing the request.
• The same action is carried out for API response.
  NOTE: The same function is used in doRequest function which is used in processing CLI requests to the Consul API.

Testing & Reproduction steps

Updated unit tests to add additional checks for content-type header
Vercel should deploy UI with no errors

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern