feat!: make transporter attribute type more generic

[forty]

Hello,

currently transporter attributes are inferred or explicitly typed as DefaultTransporter, which is too bad since the Transporter type exist I assume to make room for custom implementation (which I use, but currently I have to either extends DefaultTransporter, or force TS to ignore the typing)

[danielbankhead]

As a note - I think this is a worth-while change, but would be backwards-incompatible as Transporter does not have static USER_AGENT and the configure method (which are public for consumers). Marking this PR for a future major release.