Add custom headers to webhooks

[lenkaklimcikova]

• Brief description: Enable customers to enhance webhook notifications with custom headers.
• GitHub or Jira issue URL:
  https://kontent-ai.atlassian.net/jira/software/c/projects/DEL/boards/501?selectedIssue=EN-44
  Add custom headers to webhooks kontent-ai/management-sdk-net#258
• When do you need the code to go public? Provide a release date such as Jan 23, 2023, and/or explanation.
  Do not release before Feb 21, 2024
• Is the pull request complete and ready for review? Yes

[jancerman]

Hey @lenkaklimcikova, thanks for the PR and proposed changes to the .NET code sample. I considered using more concrete values instead of key1 and value1 but kept it as it was. I'll use more concrete examples in the API reference materials instead. :)

Merging and closing.