feat: add custom header support for resend, brevo and sendgrid

[jainpawan21]

What change does this PR introduce?

1. Add custom header support for few email providers using email overrides

Why was this change needed?

Ref:- https://twitter.com/jain_pawan21/status/1773802143956480136

Other information (Screenshots)

uses:-

await novu.events.trigger("workflowIdentifier", {
  to: "subscriberId",
  payload: {
    customKey: "customValue",
  },
  overrides: {
    email: {
      from: "[email protected]",
      headers: {
        'X-Novu-Custom-Header': 'Novu-Custom-Header-Value',
      },
    }
  },
});

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[scopsy]

💪

[pontusab]

While at it we should also add properties like reply_to https://resend.com/docs/api-reference/emails/send-email let me know if I should make a separate PR

[SokratisVidros]

@jainpawan21 What's the latest update on this PR? Shall we merge?

[jainpawan21]

@jainpawan21 What's the latest update on this PR? Shall we merge?

@SokratisVidros , yes this PR can be merged
Added reply to option in resend provider in #5389