[ios, macos] Add to-rgba expression operator. (#11725)
* [ios, macos] Add expression support to to-rgba operator.
* [ios, macos] Update style docs.
* [ios, macos] Refactored to-rgba to to-color.
* [ios, macos] Add support for to-rgba expression operator.
* [ios, macos] Add multiple parameters support to to-color operand.
* [ios, macos] Enable to-rgba operator for MGLColor or key path expressions.
* [ios, macos] Update predicates and expressions guide to reflect cast changes.
* [ios, macos] Update changelogs.
* [ios, macos] Clarify color casting usage.
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