store/remotestore: add remote store client + server support

[bradfitz]

Add an HTTP-based remote store backend so ephemeral VMs (e.g. Windows
CI with WinFSP) can use a persistent Linux server's git-backed gomodfs
as their storage backend. The ephemeral VM doesn't need internet access;
the persistent server fetches modules on demand from proxy.golang.org.

The protocol has three endpoints at the ModuleVersion level:

  GET /api/v1/modmap/{mod}/@v/{ver}  — LZ4-compressed JSON file metadata map
  GET /api/v1/file/{mod}/@v/{ver}?path=...  — raw file bytes
  GET /api/v1/meta/{mod}/@v/{ver}.{info,mod,ziphash}  — metadata files

New packages and files:

  store/remotestore/protocol.go   — shared types (ModVersionMap, FileMeta)
                                    and URL path parsing
  store/remotestore/handler.go    — server-side HTTP handler backed by an
                                    FSBackend interface (satisfied by *gomodfs.FS)
                                    with in-memory LZ4 caching per module version
  store/remotestore/remotestore.go — client-side store.Store that fetches
                                     modmaps over HTTP, derives directory structure
                                     locally, and serves Stat/Readdir with zero
                                     additional HTTP calls
  remotestore_test.go             — integration tests in the root package
  store/remotestore/remotestore_test.go — unit tests for buildDirMap and
                                          acceptsLZ4

Changes to existing code:

  gomodfs.go — add ReadOnly bool field to FS; when set, getZipRoot and
               getMetaFile return ErrCacheMiss instead of attempting
               downloads. Add exported GetZipRootOrDownload,
               GetMetaFileByExt, and GetStore helpers for the handler.
  cmd/gomodfs/gomodfs-main.go — add --remote-store=URL and
               --serve-store-api=ADDR flags.

Updates tailscale/corp#24037
Updates tailscale/corp#34812