[feature] Add fields introduced in Mastodon 4.4.0 to /api/v2/instance response (#4240)

# Description

## Checklist

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [ ] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [ ] I/we have commented the added code, particularly in hard-to-understand areas.
- [x] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [ ] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4240
Co-authored-by: nicole mikołajczyk <git@mkljczk.pl>
Co-committed-by: nicole mikołajczyk <git@mkljczk.pl>

internal/typeutils/internaltofrontend.go

@@ -1889,8 +1889,12 @@ func (c *Converter) InstanceToAPIV2Instance(ctx context.Context, i *gtsmodel.Ins
 
 	instance.Thumbnail = thumbnail
 
+	termsOfService := config.GetProtocol() + "://" + i.Domain + "/about#rules"
+
 	// configuration
 	instance.Configuration.URLs.Streaming = "wss://" + i.Domain
+	instance.Configuration.URLs.About = config.GetProtocol() + "://" + i.Domain + "/about"
+	instance.Configuration.URLs.TermsOfService = &termsOfService
 	instance.Configuration.Statuses.MaxCharacters = config.GetStatusesMaxChars()
 	instance.Configuration.Statuses.MaxMediaAttachments = config.GetStatusesMediaMaxFiles()
 	instance.Configuration.Statuses.CharactersReservedPerURL = instanceStatusesCharactersReservedPerURL
@@ -1936,6 +1940,8 @@ func (c *Converter) InstanceToAPIV2Instance(ctx context.Context, i *gtsmodel.Ins
 	instance.Registrations.Enabled = config.GetAccountsRegistrationOpen()
 	instance.Registrations.ApprovalRequired = true // always required
 	instance.Registrations.Message = nil           // todo: not implemented
+	instance.Registrations.MinAge = nil            // not implemented
+	instance.Registrations.ReasonRequired = config.GetAccountsReasonRequired()
 
 	// contact
 	instance.Contact.Email = i.ContactEmail

internal/typeutils/internaltofrontend_test.go

@@ -2521,7 +2521,10 @@ func (suite *InternalToFrontendTestSuite) TestInstanceV2ToFrontend() {
   ],
   "configuration": {
     "urls": {
-      "streaming": "wss://localhost:8080"
+      "streaming": "wss://localhost:8080",
+      "about": "http://localhost:8080/about",
+      "privacy_policy": null,
+      "terms_of_service": "http://localhost:8080/about#rules"
     },
     "accounts": {
       "allow_custom_css": true,
@@ -2587,7 +2590,9 @@ func (suite *InternalToFrontendTestSuite) TestInstanceV2ToFrontend() {
   "registrations": {
     "enabled": true,
     "approval_required": true,
-    "message": null
+    "message": null,
+    "min_age": null,
+    "reason_required": true
   },
   "contact": {
     "email": "admin@example.org",