feat: Implement feature flag creation

[inourbubble2]

• Implement feature flag creation and add unit tests
• Add spring-security-test dependency
• Configure JPA settings
• Modify JsonConverter to throw IllegalArgumentException on failure
• Disable OSIV)

I haven't run ktlint yet since there are many ongoing PRs in backend to avoid conflict with #39. It would be better to run ktlint all at once after this PR and #39 are merged.

Example response of the API:

HTTP/1.1 200 
Content-Type: application/json

{
  "status": "Success",
  "message": "Created flag successfully",
  "data": {
    "key": "user-limit",
    "status": true,
    "type": "number",
    "defaultValue": {
      "number": "10"
    },
    "description": "description",
    "variants": [
      {
        "number": "10"
      },
      {
        "free": "100"
      }
    ],
    "createdBy": "username",
    "updatedBy": "username",
    "createdAt": "2025-02-17T00:27:06.122584Z",
    "updatedAt": "2025-02-17T00:27:06.161132Z"
  }
}

[eunhwa99]

👍

[eunhwa99]

Good job