Reduce memory usage of MatchSearch

```bash

$ benchstat f8aa6f9.txt new.txt
goos: darwin
goarch: arm64
pkg: github.com/gravitational/teleport/lib/auth
                                        │ f8aa6f9.txt │              new.txt               │
                                        │                    sec/op                    │   sec/op     vs base               │
ListUnifiedResources/labels-12                                             161.1m ± 4%   168.7m ± 5%  +4.73% (p=0.043 n=10)
ListUnifiedResources/predicate_path-12                                     383.6m ± 1%   389.4m ± 1%  +1.50% (p=0.000 n=10)
ListUnifiedResources/predicate_index-12                                    396.6m ± 1%   397.7m ± 3%       ~ (p=0.190 n=10)
ListUnifiedResources/search_lower-12                                       258.6m ± 2%   266.6m ± 3%  +3.07% (p=0.007 n=10)
ListUnifiedResources/search_upper-12                                       266.1m ± 8%   259.2m ± 2%       ~ (p=0.052 n=10)
geomean                                                                    278.9m        282.7m       +1.36%

                                        │ f8aa6f9.txt │               new.txt                │
                                        │                     B/op                     │     B/op      vs base                │
ListUnifiedResources/labels-12                                            25.93Mi ± 0%   25.94Mi ± 0%        ~ (p=0.089 n=10)
ListUnifiedResources/predicate_path-12                                    264.1Mi ± 0%   264.1Mi ± 0%        ~ (p=0.853 n=10)
ListUnifiedResources/predicate_index-12                                   264.1Mi ± 0%   264.1Mi ± 0%        ~ (p=0.927 n=10)
ListUnifiedResources/search_lower-12                                      73.94Mi ± 0%   55.63Mi ± 0%  -24.77% (p=0.000 n=10)
ListUnifiedResources/search_upper-12                                      74.84Mi ± 0%   56.53Mi ± 0%  -24.46% (p=0.000 n=10)
geomean                                                                   100.0Mi        89.33Mi       -10.68%

                                        │ f8aa6f9.txt │               new.txt               │
                                        │                  allocs/op                   │  allocs/op   vs base                │
ListUnifiedResources/labels-12                                             911.7k ± 0%   911.7k ± 0%        ~ (p=0.469 n=10)
ListUnifiedResources/predicate_path-12                                     5.862M ± 0%   5.862M ± 0%   -0.00% (p=0.019 n=10)
ListUnifiedResources/predicate_index-12                                    5.712M ± 0%   5.712M ± 0%        ~ (p=0.305 n=10)
ListUnifiedResources/search_lower-12                                       1.962M ± 0%   1.662M ± 0%  -15.29% (p=0.000 n=10)
ListUnifiedResources/search_upper-12                                       2.112M ± 0%   1.812M ± 0%  -14.20% (p=0.000 n=10)
geomean                                                                    2.633M        2.470M        -6.18%
```

api/types/resource.go

@@ -544,15 +544,21 @@ func IsValidLabelKey(s string) bool {
 // Returns true if all search vals were matched (or if nil search vals).
 // Returns false if no or partial match (or nil field values).
 func MatchSearch(fieldVals []string, searchVals []string, customMatch func(val string) bool) bool {
-	// Case fold all values to avoid repeated case folding while matching.
-	caseFoldedSearchVals := utils.ToLowerStrings(searchVals)
-	caseFoldedFieldVals := utils.ToLowerStrings(fieldVals)
+	caseFoldedFieldVals := make(map[string]string, len(fieldVals))
 
 Outer:
-	for _, searchV := range caseFoldedSearchVals {
+	for _, searchV := range searchVals {
+		searchV = strings.ToLower(searchV)
+
 		// Iterate through field values to look for a match.
-		for _, fieldV := range caseFoldedFieldVals {
-			if strings.Contains(fieldV, searchV) {
+		for _, fieldV := range fieldVals {
+			f, ok := caseFoldedFieldVals[fieldV]
+			if !ok {
+				f = strings.ToLower(fieldV)
+				caseFoldedFieldVals[fieldV] = f
+			}
+
+			if strings.Contains(f, searchV) {
 				continue Outer
 			}
 		}