feat(multipart): add Form::set_boundary for custom boundaries

[sqdshguy]

Adds Form::set_boundary(&mut self, boundary: impl Into<String>) so users can specify their own multipart boundary string (e.g. ------WebKitFormBoundary... for browser emulation). This is useful for matching browser fingerprints during emulation, since the default gen_boundary() output doesn't resemble any real browser.

API and doc comment follow reqwest's approach in seanmonstar/reqwest#2814. Includes unit and integration tests.

Closes #1030

[gemini-code-assist]

Summary of Changes

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This pull request enhances the multipart form functionality by providing a mechanism for users to specify custom boundary strings. This capability is vital for use cases such as browser emulation, where precise control over the multipart boundary is necessary to mimic specific client behaviors. The implementation includes clear documentation regarding the potential risks of using custom boundaries and is thoroughly tested to ensure its reliability.

Highlights

• Feature Addition: Introduced a new public method, Form::set_boundary, allowing users to explicitly define the multipart boundary string.
• Use Case: This feature is primarily intended for advanced scenarios like browser emulation, where matching specific browser-generated boundary fingerprints is crucial.
• Risk Mitigation: The documentation for set_boundary includes a prominent warning about the significant risk of generating corrupted bodies when using custom boundaries, advising users to proceed with caution.
• Testing: Comprehensive testing has been added, including both a unit test in src/client/multipart.rs and an integration test in tests/multipart.rs, to ensure the correct application and behavior of custom boundaries.

Changelog

• src/client/multipart.rs
  • Added pub fn set_boundary(&mut self, boundary: impl Into<String>) to the Form struct.
  • Included a detailed doc comment for set_boundary explaining its purpose and warning about potential risks.
  • Added a unit test custom_boundary_is_applied to verify the functionality of setting a custom boundary.
• tests/multipart.rs
  • Added an integration test text_part_with_custom_boundary to validate that custom boundaries are correctly used in a full request-response cycle.

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[gemini-code-assist]

Code Review

This pull request adds the ability to set a custom boundary for multipart forms, which is a useful feature for browser emulation. The implementation is straightforward and follows the pattern of similar APIs in other libraries. The addition of both unit and integration tests is great. I've found one issue in the new integration test related to how a multi-line string is defined, which would likely cause the test to fail. I've left a suggestion to fix it. This seems to be a pattern copied from other tests, so it might be worth fixing it in other places as well.

[0x676e67]

I prefer the "with_boundary" initialization method over "set_boundary".

    pub fn with_boundary(boundary: impl Into<String>) -> Form {
        let mut inner = FormParts::new();
        inner.boundary = boundary;
        Form { inner }
    }

[sqdshguy]

I prefer the "with_boundary" initialization method over "set_boundary".

    pub fn with_boundary(boundary: impl Into<String>) -> Form {
        let mut inner = FormParts::new();
        inner.boundary = boundary;
        Form { inner }
    }

Done. Let me know if there's something else to change. First time contributing here, so not sure about the conventions