IHeaders type definition

[Pipo93]

Issue

Currently IHeaders are defined to contain strings or Buffers:

kafkajs/types/index.d.ts

Lines 98 to 100 in 4139127

	export interface IHeaders {
	[key: string]: Buffer | string
	}

When consuming a message within a typescript project this may cause some issues when accessing header keys which are not available, e.g.: const testHeader = headers?.test.toString().
In this case typescript wouldn't complain but the application will crash when test header is not set.

Suggested solution:

IHeader keys should also be typed with undefined:

export interface IHeaders {
  [key: string]: Buffer | string | undefined
}