config: add support for downstream TLS server name (#3243)

* config: add support for downstream TLS server name

* fix whitespace

* fix whitespace

* add docs

* add tls_upstream_server_name and tls_downstream_server_name to config

* Update docs/reference/settings.yaml

Co-authored-by: Alex Fornuto <afornuto@pomerium.com>

* Update docs/reference/readme.md

Co-authored-by: Alex Fornuto <afornuto@pomerium.com>

* add deprecation notice

Co-authored-by: Alex Fornuto <afornuto@pomerium.com>

internal/sets/sorted.go

@@ -0,0 +1,65 @@
+package sets
+
+import "github.com/google/btree"
+
+type stringItem string
+
+func (item stringItem) Less(than btree.Item) bool {
+	return item < than.(stringItem)
+}
+
+// A SortedString is a set of strings with sorted iteration.
+type SortedString struct {
+	b *btree.BTree
+}
+
+// NewSortedString creates a new sorted string set.
+func NewSortedString() *SortedString {
+	return &SortedString{
+		b: btree.New(8),
+	}
+}
+
+// Add adds a string to the set.
+func (s *SortedString) Add(elements ...string) {
+	for _, element := range elements {
+		s.b.ReplaceOrInsert(stringItem(element))
+	}
+}
+
+// Clear clears the set.
+func (s *SortedString) Clear() {
+	s.b = btree.New(8)
+}
+
+// Delete deletes an element from the set.
+func (s *SortedString) Delete(element string) {
+	s.b.Delete(stringItem(element))
+}
+
+// ForEach iterates over the set in ascending order.
+func (s *SortedString) ForEach(callback func(element string) bool) {
+	s.b.Ascend(func(i btree.Item) bool {
+		return callback(string(i.(stringItem)))
+	})
+}
+
+// Has returns true if the elment is in the set.
+func (s *SortedString) Has(element string) bool {
+	return s.b.Has(stringItem(element))
+}
+
+// Size returns the size of the set.
+func (s *SortedString) Size() int {
+	return s.b.Len()
+}
+
+// ToSlice returns a slice of all the elements in the set.
+func (s *SortedString) ToSlice() []string {
+	arr := make([]string, 0, s.Size())
+	s.b.Ascend(func(i btree.Item) bool {
+		arr = append(arr, string(i.(stringItem)))
+		return true
+	})
+	return arr
+}