feat: add timestamp

[msstoci]

add timestamp on Embeds

reference: https://birdie0.github.io/discord-webhooks-guide/structure/embed/timestamp.html

[gtuk]

@msstoci Thanks for you contribution, it's really appreciated. I have question though, based on https://birdie0.github.io/discord-webhooks-guide/discord_webhook.html the timestamp should be ISO8601. I am not sure the timestamp coming from golang is in this format, could you provide me with more info/details here

[attributeerror]

// MarshalJSON implements the json.Marshaler interface.
// The time is a quoted string in the RFC 3339 format with sub-second precision.
// If the timestamp cannot be represented as valid RFC 3339
// (e.g., the year is out of range), then an error is reported.
func (t Time) MarshalJSON() ([]byte, error) {
	b := make([]byte, 0, len(RFC3339Nano)+len(`""`))
	b = append(b, '"')
	b, err := t.appendStrictRFC3339(b)
	b = append(b, '"')
	if err != nil {
		return nil, errors.New("Time.MarshalJSON: " + err.Error())
	}
	return b, nil
}

Pulled from Go v1.20.7 source code, looks like time.Time is serialised into a string that matches the RFC3339 (which is very similar to ISO 8601 - main difference is decimal fractions of the smallest time values & case sensitivity, but this seems to be fine when sending this value to Discord.

cc @gtuk

[TheFiordi]

@msstoci Hi. the timestamp field should be added to the Embed struct and not to Message

type Message struct {
	Username        *string          `json:"username,omitempty"`
	AvatarUrl       *string          `json:"avatar_url,omitempty"`
	Content         *string          `json:"content,omitempty"`
	Embeds          *[]Embed         `json:"embeds,omitempty"`
	AllowedMentions *AllowedMentions `json:"allowed_mentions,omitempty"`
}

type Embed struct {
	Title       *string    `json:"title,omitempty"`
	Url         *string    `json:"url,omitempty"`
	Description *string    `json:"description,omitempty"`
	Color       *string    `json:"color,omitempty"`
	Author      *Author    `json:"author,omitempty"`
	Fields      *[]Field   `json:"fields,omitempty"`
	Thumbnail   *Thumbnail `json:"thumbnail,omitempty"`
	Image       *Image     `json:"image,omitempty"`
	Footer      *Footer    `json:"footer,omitempty"`
	Timestamp   *time.Time `json:"timestamp,omitempty"` <------- Here
}

[attributeerror]

@TheFiordi Appreciate the review, and you're totally right, but the PR was raised by @msstoci :)

[TheFiordi]

@TheFiordi Appreciate the review, and you're totally right, but the PR was raised by @msstoci :)

You're right, I'm so sorry!!

[gtuk]

@msstoci do you want to update your PR, with the suggestion from @TheFiordi