Library should allow for a custom HttpMessageHandler

[Arlodotexe]

Problem

The library doesn't allow for custom HttpMessageHandlers. That means no custom headers, no custom functionality, and no polyfil on certain places where it's needed.

Solution

Rather than constructing a new HttpClientHandler here:
https://github.com/ipfs-shipyard/net-ipfs-http-client/blob/main/src/IpfsClient.cs#L223

We should make this a HttpMessageHandler and move it to a property on the IpfsClient class, where the default value is new HttpClientHandler().