feat(transport): support dynamic headers in transports

[rbaradari]

Why

Currently, the Faro Web SDK only allows to configure static headers for transport requests.

When sending data through a proxy it might require a valid authentication token (e.g. JWT). Since tokens can expire and need to be refreshed, we need a way to lookup dynamic header values for each request.

What

Extended the RequestOptions headers to also support callback functions that return a header value. This change is implemented for the Fetch transport and to stay aligned also for the OLTP HTTP transport.

I've not opted to also implement a Promise based asynchronous lookup. That would have significantly increased the change set. It is expected that any token refreshes are done by the application. When building requests by Faro we can directly lookup the currently available token.

Links

Addresses feature request #1490.

Checklist

• Tests added
• Changelog updated
• Documentation updated

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