support headers for TSA calls

[n8fr8]

In order to support a non-free Timestamping Authority (TSA), there needs to be support for setting a header for calls to the configured TSA, as outline in the Trufo.ai documentation here for example:

Authentication
Auth Header
X-API-Key: <YOUR_TSA_API_KEY>
Description
All timestamp requests must include a valid TSA API key in the X-API-Key header. You can create and manage your API keys in the section below.

The core c2pa-rs Signer does support header options, but I am not sure if this is exposed via the C API and is definitely not available in Kotlin/Android

[redaranj]

@n8fr8 This feature is not exposed in the C API yet, but we will add it when it is.