Add annotation-based custom field attributes support

[ekoops]

This PR adds support for specifying custom field attributes through annotations.

Given the following input,

struct S {
    /// <div rustbindgen attribute="#[Attr1] #[Attr2]" attribute="#[Attr3(value)]"></div>
    int field_1;
    char field_2;
};

the generated bindings will contain the specified attributes

pub struct S {
    /// <div rustbindgen attribute="#[Attr1] #[Attr2]" attribute="#[Attr3(value)]"></div>
    #[Attr1]
    #[Attr2]
    #[Attr3(value)]
    pub field_1: ::std::os::raw::c_int,
    pub field_2: ::std::os::raw::c_char,
}

[ekoops]

r? @emilio

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[ekoops]

Hey! Is there any chance this patch will get considered any soon?