feat: SlickVPN Support (#961)

- `internal/updater/html` package
- Add unit tests for slickvpn updating code
- Change shared html package to be more share-able
- Split html utilities in multiple files
- Fix processing .ovpn files with prefix space

Authored by @Rohaq 
Co-authored-by: Quentin McGaw <quentin.mcgaw@gmail.com>

internal/provider/slickvpn/updater/servers.go

@@ -0,0 +1,82 @@
+package updater
+
+import (
+	"context"
+	"fmt"
+	"sort"
+
+	"github.com/qdm12/gluetun/internal/constants/vpn"
+	"github.com/qdm12/gluetun/internal/models"
+	"github.com/qdm12/gluetun/internal/provider/common"
+	"github.com/qdm12/gluetun/internal/updater/openvpn"
+)
+
+func (u *Updater) FetchServers(ctx context.Context, minServers int) (
+	servers []models.Server, err error) {
+	hostToData, err := fetchServers(ctx, u.client)
+	if err != nil {
+		return nil, fmt.Errorf("fetching and parsing website: %w", err)
+	}
+
+	openvpnURLs := make([]string, 0, len(hostToData))
+	for _, data := range hostToData {
+		openvpnURLs = append(openvpnURLs, data.ovpnURL)
+	}
+
+	if len(openvpnURLs) < minServers {
+		return nil, fmt.Errorf("%w: %d and expected at least %d",
+			common.ErrNotEnoughServers, len(openvpnURLs), minServers)
+	}
+
+	const failEarly = false // some URLs from the website are not valid
+	udpHostToURL, errors := openvpn.FetchMultiFiles(ctx, u.client, openvpnURLs, failEarly)
+	for _, err := range errors {
+		u.warner.Warn(fmt.Sprintf("fetching OpenVPN files: %s", err))
+	}
+
+	if len(udpHostToURL) < minServers {
+		return nil, fmt.Errorf("%w: %d and expected at least %d",
+			common.ErrNotEnoughServers, len(udpHostToURL), minServers)
+	}
+
+	hosts := make([]string, 0, len(udpHostToURL))
+	for host := range udpHostToURL {
+		hosts = append(hosts, host)
+	}
+
+	resolveSettings := parallelResolverSettings(hosts)
+	hostToIPs, warnings, err := u.parallelResolver.Resolve(ctx, resolveSettings)
+	for _, warning := range warnings {
+		u.warner.Warn(warning)
+	}
+	if err != nil {
+		return nil, fmt.Errorf("resolving hosts: %w", err)
+	}
+
+	if len(hostToIPs) < minServers {
+		return nil, fmt.Errorf("%w: %d and expected at least %d",
+			common.ErrNotEnoughServers, len(hosts), minServers)
+	}
+
+	servers = make([]models.Server, 0, len(hostToIPs))
+	for host, IPs := range hostToIPs {
+		_, udp := udpHostToURL[host]
+
+		serverData := hostToData[host]
+
+		server := models.Server{
+			VPN:      vpn.OpenVPN,
+			Region:   serverData.region,
+			Country:  serverData.country,
+			City:     serverData.city,
+			Hostname: host,
+			UDP:      udp,
+			IPs:      IPs,
+		}
+		servers = append(servers, server)
+	}
+
+	sort.Sort(models.SortableServers(servers))
+
+	return servers, nil
+}