[feature] Support incoming avatar/header descriptions (#4275)

Signed-off-by: nicole mikołajczyk <git@mkljczk.pl>

# Description

Follow-up to #4270

Closes https://codeberg.org/superseriousbusiness/gotosocial/issues/3450

## 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.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [ ] I/we have made any necessary changes to documentation.
- [x] I/we have added tests that cover new code.
- [x] 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/4275
Co-authored-by: nicole mikołajczyk <git@mkljczk.pl>
Co-committed-by: nicole mikołajczyk <git@mkljczk.pl>

testrig/transportcontroller.go

@@ -566,6 +566,17 @@ func WebfingerResponse(req *http.Request) (
 				},
 			},
 		}
+	case "https://shrimpnet.example.org/.well-known/webfinger?resource=acct%3Ashrimp%40shrimpnet.example.org":
+		wfr = &apimodel.WellKnownResponse{
+			Subject: "acct:shrimp@shrimpnet.example.org",
+			Links: []apimodel.Link{
+				{
+					Rel:  "self",
+					Type: applicationActivityJSON,
+					Href: "https://shrimpnet.example.org/users/shrimp",
+				},
+			},
+		}
 	case "https://misconfigured-instance.com/.weird-webfinger-location/webfinger?resource=acct%3Asomeone%40misconfigured-instance.com":
 		wfr = &apimodel.WellKnownResponse{
 			Subject: "acct:someone@misconfigured-instance.com",