Support string matcher for header matching in RBAC.

[zasweq]

Java and C share Header Matcher code between RBAC and xDS (used in the route table). Thus, other languages do support string matching for header matching in RBAC. gRPC-Go currently does not:

grpc-go/internal/xds/rbac/matchers.go

Line 267 in f2fbb0e

func newHeaderMatcher(headerMatcherConfig *v3route_componentspb.HeaderMatcher) (*headerMatcher, error) {

. Alongside supporting string matching, we should look into trying to share header matching code between xDS and RBAC like other languages (or perhaps there was a technical reason we decided not to).