hooks: Recursively search embedded fields for methods

[abhinav]

Follow up to #493 and 840220c

Kong currently supports hooks on embedded fields of a parsed node,
but only at the first level of embedding:

type mainCmd struct {
    FooOptions
}

type FooOptions struct {
    BarOptions
}

func (f *FooOptions) BeforeApply() error {
    // this will be called
}

type BarOptions struct {
}

func (b *BarOptions) BeforeApply() error {
    // this will not be called
}

This change adds support for hooks to be defined
on embedded fields of embedded fields so that the above
example would work as expected.

Per #493, the definition of "embedded" field is adjusted to mean:

• Any anonymous (Go-embedded) field that is exported
• Any non-anonymous field that is tagged with embed:""

Testing:
Includes a test case for embedding an anonymous field in an embed:""
and an embed:"" field in an anonymous field.

[abhinav]

One thing worth noting:
Adding a receivers parameter here allows the recursive calls to reuse the slice's backing storage
so that when one append gives the slice 2x capacity, that is re-used in following appends
before needing another resize further down the stack.

We could change the nil below to something like make([]reflect.Value, 0, 16), and this iteration/collection would be alloc-free for a recursive depth of 16 (which should be more than enough for 90% of the use cases).

[alecthomas]

Nice, thanks!