Add multipart form data support

[tobischo]

Currently, file uploads are not supported through grac.
This adds support for multipart form data. It relies on the support for it provided by typhoeus

[Copilot]

Pull request overview

Adds multipart/form-data support to Grac::Client so callers can upload files using Typhoeus’ multipart handling.

Changes:

• Detect multipart/form-data requests, set Typhoeus’ multipart: true, and remove the Content-Type header so Typhoeus can set the boundary.
• Update request body preparation to avoid encoding multipart bodies.
• Add specs and documentation, and bump gem version to 4.5.0.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
lib/grac/client.rb	Adds multipart request handling + multipart body preparation branch.
spec/lib/grac/client_spec.rb	Adds coverage for multipart/non-multipart call behavior and multipart body preparation.
README.md	Documents multipart/file upload usage with examples.
lib/grac/version.rb	Bumps library version to 4.5.0.
Gemfile.lock	Updates locked version to 4.5.0.
CHANGELOG.md	Adds a 4.5.0 entry describing multipart support.

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