Use typed Url to construct XRPC URI #1

this probably either does too much or too little:
- could have fixed the double-slash thing by stripping trailing slash from the base uri or,
- if you like typed URLs, it probably makes sense to do a bigger refactor toward them.

...sidestepping adding the actual `url` dependency by using the one included with `reqwest`. maybe this is not what you want.

currently using `EncodeError::Other` instead of adding a variant to `EncodeError` itself because the parse error comes from `Url`, which we snuck in from `reqwest`, which isn't a dependency of the `-common` crate.