Add application settings to allowlist integrations

What does this MR do and why?

Adds application settings to support the allow list integrations feature #500610 (closed).

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

GET application/settings

curl --location 'http://gdk.test:3000/api/v4/application/settings' \
--header 'Authorization: Bearer <PAT>'

Response should include:

"allow_all_integrations": true,
"allowed_integrations": [],

PUT application/settings

Add two integration names to allowed_integrations:

curl --location --globoff --request PUT 'http://gdk.test:3000/api/v4/application/settings?allowed_integrations[]=jira&allowed_integrations[]=foo' \
--header 'Authorization: Bearer <PAT>'

Response should include:

"allowed_integrations": [
    "jira",
    "slack"
],

Attempting to add an invalid integration name should fail:

curl --location --globoff --request PUT 'http://gdk.test:3000/api/v4/application/settings?allowed_integrations[]=foo&allowed_integrations[]=slack' \
--header 'Authorization: Bearer <PAT>'

Response should be:

{
    "message": {
        "allowed_integrations": [
            "contains unknown integration names"
        ]
    }
}

Related to #500610 (closed)