omitempty for NULL attribute values from custom marshalers

[babattles]

Adds an EncoderOption that allows the encoder to omit NULL attribute values when marshaling a field tagged with omitempty.

Related issues:

• Custom Marshaler returning NULL should be omitted from .MarshalMap() with omitempty tag #2731
• Add support for omitting attribute values returned by custom Marshaler implementations #1429

[lucix-aws]

Once CI passes I'm going to push a clarification to the doc string on this new field, since omitempty does chop out NULLs for values in the normal marshaling workflow e.g.

type Foo struct {
	A string
	B *string `dynamodbav:",omitempty"` // will get omitted today if B == nil
}

Basically it'll be something like

	// When enabled, the encoder will omit null (NULL) attribute values returned
	// from custom marshalers tagged with `omitempty`.