Allow for server migrations

[didimitrie]

Step 0:

• I've read the contribution guidelines!

This is a bigger issue, but we should expose the ability to change the baseUrl on a given api client. This should perhaps be correlated with an account change/migration which can trigger subsequent changes on deserialisation in the end clients (gh/dynamo/rhino/etc.)

Needless to say, this affects SpeckleRhino, SpeckleDynamo and SpeckleRevit.