fix(rust,client): convert form isArray values to string

[PiDelport]

• Tracking issue: [BUG][RUST] Twilio OpenAPI codegen errors #10421

The Rust reqwest client's form handling fails to call to_string on array params, which fails to compile for arrays of values other than String.

This adds the missing to_string calls (similar to what's done with queryParams earlier in the function).

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[PiDelport]

(Rebased)

[wing328]

LGTM but I've not tested it locallly.