Add support for HTTP rewrites (#1062)

This adds a new configuration option for the HTTP client to replace URI prefixes when making outbound calls.

Follows the design of https://github.com/apple/pkl-evolution/pull/17

pkl-server/src/test/kotlin/org/pkl/server/ServerMessagePackCodecTest.kt

@@ -96,6 +96,7 @@ class ServerMessagePackCodecTest {
           Http(
             proxy = Proxy(URI("http://foo.com:1234"), listOf("bar", "baz")),
             caCertificates = byteArrayOf(1, 2, 3, 4),
+            rewrites = mapOf("https://foo.com" to "https://bar.com"),
           ),
         externalModuleReaders = mapOf("external" to externalReader, "external2" to externalReader),
         externalResourceReaders = mapOf("external" to externalReader),