Add support for meta annotations on model fields

[codyaray]

Instead of using the JSONAPIMeta() hook, allow specifying metadata directly in the model/node.

type Blog struct {
    ID              int       `jsonapi:"primary,blogs"`
    Title           string    `jsonapi:"attr,title"`
    ModifiedAt      time.Time `jsonapi:"meta,modified_at"` // not iso 8601, serialize as epoch time
    DeletedAt       time.Time `jsonapi:"meta,deleted_at,iso8601"`
    ResourceVersion string    `jsonapi:"meta,resource_version"`
}

This supports epoch timestamps, iso8601 datetime strings, and other stringify-able data types.

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