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-core/src/test/files/LanguageSnippetTests/input/errors/analyzeInvalidHttpModule.pkl

@@ -1,3 +0,0 @@
-import "pkl:analyze"
-
-result = analyze.importGraph(Set("http://localhost:0/foo.pkl"))

pkl-core/src/test/files/LanguageSnippetTests/output/errors/analyzeInvalidHttpModule.err

@@ -1,14 +0,0 @@
-–– Pkl Error ––
-HTTP/1.1 header parser received no bytes
-
-x | result = analyze.importGraph(Set("http://localhost:0/foo.pkl"))
-             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-at analyzeInvalidHttpModule#result (file:///$snippetsDir/input/errors/analyzeInvalidHttpModule.pkl)
-
-xxx | text = renderer.renderDocument(value)
-             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-at pkl.base#Module.output.text (pkl:base)
-
-xxx | bytes = text.encodeToBytes("UTF-8")
-              ^^^^
-at pkl.base#Module.output.bytes (pkl:base)