Add Authentication Options for DAST API On-Demand screens

What does this MR do and why?

1. Currently, from UI we are only setting the username and password when the target_type is website.
2. The changes will allow the users to set the username and password when the target_type is api.
3. The above 2 operations can be done by Graphql mutations but do not have any use case in case of target_type api.
4. On Create action and on supplying target_type api we are setting DAST_API_HTTP_PASSWORD_BASE64 ENV and DAST_API_HTTP_USERNAME username.

Note: For API basic auth we don't need target_url and other auth options. We only need username and password from auth options

Testing the changes:

1. Sample Graphql that can be used for testing.

mutation($dastSiteProfileUpdateInput: DastSiteProfileUpdateInput!) {
  dastSiteProfileUpdate(input: $dastSiteProfileUpdateInput) {
    clientMutationId
    errors
    id
  }
}


{
   "dastSiteProfileUpdateInput":{
      "fullPath":"root/dast-test1",
      "id":"gid://gitlab/DastSiteProfile/3",
      "profileName":"profile2",
      "targetUrl":"https://gitlab.com/",
      "auth":{
         "enabled":false, "username": "test2", "password": "pass2"
      }
   }
}

mutation($dastSiteProfileCreateInput: DastSiteProfileCreateInput!) {
  dastSiteProfileCreate(input: $dastSiteProfileCreateInput) {
    clientMutationId
    errors
    id
  }
}

{
   "dastSiteProfileCreateInput":{
      "fullPath":"gitlab-org/go-fuzzing-example8",
      "profileName":"Open-source intermediate methodology",
      "targetUrl":"http://example18.test",
      "excludedUrls":"http://example18.test/signout",
      "requestHeaders":"Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:47.0) Gecko/20100101 Firefox/47.0",
      "auth":{
         "enabled":true,
        "submitField": "css:button[type=\"submit\"]"
      }
   }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #350514 (closed)