Update uniffi-rs to 0.29

[0Xellos]

Summary of changes:

• bump uniffi-rs version to 0.29
• bump its dependencies that are used for this bindgen: askama -> 0.13, cargo_metadata -> 0.19
• remove cases for uniffi_bindgen::interface::Type::External since this enum value got removed
• add a case for uniffi_bindgen::interface::FfiType::MutReference (identical to Reference) since this enum value got added
• rename filter deref to cpp_deref, it was causing conflict with a new builtin filter
• changed uniffi_bindgen::backend::TemplateExpression to String since this type got removed
• added impl CustomTypesConfig; changed the use of into_custom and from_custom in custom.cpp template to lift and lower defined here
• cast some names from String to &str
• renamed iter_types to iter_local_types on ComponentInterface since that method got removed - this is what I'm least confident about in this PR, there are multiple methods returning type iterators...
• renamed variable fn to func because of conflict with the Rust keyword
• changed how filters get_*_definition are used: ci|get_*_definition(name) -> ci.get_*_definition(name).unwrap()
• broke up filter chain {%- let ffi_converter_name = typ|ffi_converter_name|class_name %} in enum.hpp and err.hpp templates into 2 steps, was required to pass String as &str
• rewrite custom type definitions in fixtures to use macro uniffi::custom_newtype

Disclaimer: I'm a C++ guy, not a Rust guy. I was mostly going by a similar update that was done in uniffi-bindgen-cs.

[Lipt0nas]

The MR looks good overall, thank you for contributing!

[tomas-krupa]

@Lipt0nas Hello, do you plan to merge this update in the near future? Thanks a lot.

[Lipt0nas]

Everything looks in order, thanks for the patience and contribution 🙂