refactor(storage): new storage file structure

- new directory structure containing manifest.json and one json file per provider, by default.
- the manifest.json file can specify a filepath for each vpn provider
- each vpn provider json data file can contain the `"preferred": true` field to enforce it is used even if outdated, unless there is a version mismatch
- `STORAGE_SERVERS_DIRECTORY_PATH` replaces `STORAGE_FILEPATH` (which is now a migration source only). It sets the directory where server manifest and per-provider JSON files are stored (default: `/gluetun/servers/`).
- First-run migration: On startup, gluetun checks for the old /gluetun/servers.json file; if found and no new manifest exists, it automatically migrates all data to /gluetun/servers/ directory structure
- Silent fallback: If legacy file isn't found, uses the new directory path normally
- Legacy cleanup: After successful migration, attempts to remove the old fat JSON file (logs warning only if removal fails, e.g., read-only bind mounts)

gluetun-servers/pkg/publicip/api/api_test.go

@@ -0,0 +1,64 @@
+package api
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func Test_ParseProvider(t *testing.T) {
+	t.Parallel()
+
+	testCases := map[string]struct {
+		s          string
+		provider   Provider
+		errMessage string
+	}{
+		"empty": {
+			errMessage: `API name is not valid: "" can only be ` +
+				`"cloudflare", "ifconfigco", "ip2location", "ipinfo" or a custom echoip# url`,
+		},
+		"invalid": {
+			s: "xyz",
+			errMessage: `API name is not valid: "xyz" can only be ` +
+				`"cloudflare", "ifconfigco", "ip2location", "ipinfo" or a custom echoip# url`,
+		},
+		"ipinfo": {
+			s:        "ipinfo",
+			provider: IPInfo,
+		},
+		"IpInfo": {
+			s:        "IpInfo",
+			provider: IPInfo,
+		},
+		"echoip_url_empty": {
+			s: "echoip#",
+			errMessage: `echoip# custom URL is not valid: "" ` +
+				`does not match regular expression: ^http(s|):\/\/.+$`,
+		},
+		"echoip_url_invalid": {
+			s: "echoip#postgres://localhost:3451",
+			errMessage: `echoip# custom URL is not valid: "postgres://localhost:3451" ` +
+				`does not match regular expression: ^http(s|):\/\/.+$`,
+		},
+		"echoip_url_valid": {
+			s:        "echoip#http://localhost:3451",
+			provider: Provider("echoip#http://localhost:3451"),
+		},
+	}
+
+	for name, testCase := range testCases {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			provider, err := ParseProvider(testCase.s)
+
+			assert.Equal(t, testCase.provider, provider)
+			if testCase.errMessage != "" {
+				assert.EqualError(t, err, testCase.errMessage)
+			} else {
+				assert.NoError(t, err)
+			}
+		})
+	}
+}