feat(esp_encrypted_img): feature added to get header size which helps… (IEC-45)

[nileshkale123]

Checklist

• [x] Component contains License
• [x] Component contains README.md
• [x] Component contains idf_component.yml file with url field defined
• [x] Component was added to upload job
• [x] Component was added to build job

Change description

This MR added the API to get the header size which helps in decryption during pre_encrypted_ota. This header size is helpful to check whether the complete image has been received or not during pre_encypted_ota and partial HTTP download config is set.

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