Multipart form-data body builder

[OmarAlJarrah]

Summary

Adds a formdata package — an ergonomic builder over mime/multipart for
multipart/form-data request bodies (file uploads), producing a replayable
body so retries can re-send it.

What's included

• Form — chainable, deferred-error builder: Field(name, value),
  File(field, filename, r), FileBytes(field, filename, data). The first error
  is reported at Build; no panics.
• ContentType() — the multipart/form-data; boundary=… content type.
• Build() (*bytes.Reader, error) — finalizes the form into an in-memory,
  replayable body (so http.NewRequest populates GetBody).
• NewRequest(ctx, method, url) — the ergonomic entry point: returns a ready
  *http.Request with the multipart Content-Type set and a replayable body.

Background

This was one of the items deferred from the HTTP-value-types roadmap subsystem.
It buffers the body in memory (the documented default) so it survives the retry
policy's body replay; streaming non-replayable uploads remain out of scope.

Behavior changes

None — new package only.

Test plan

• go build ./...
• go vet ./...
• gofmt -l . (clean)
• go test -race ./... (all packages pass)
• Covered: build a field + file and parse it back with mime/multipart (field
  value and file name/content round-trip); NewRequest sets the boundary
  Content-Type and yields a replayable body (read via Body then GetBody); a
  failing file reader surfaces at Build; build-twice and field-after-build
  error; an empty form builds a valid body.

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