Support the creation of a new mutable object with a pre-determined signature key

[exarkun]

Sometimes you want to create or re-create a specific mutable object.

Two examples:

• You are writing a compliance test suite and you want to verify certain test vectors are handled correctly.
• You are embedding a mutable capability into an external backup system which you want to initialize _before_ you have uploaded anything to a storage server.

Currently this is not possible because the mutable creation APIs all randomly generate a new RSA key and use that. If you try to write the compliance test suite, all your mutable capabilities come out different each time because they each have a new random RSA key. If you want to embed a capability in an external system, you must create it first because otherwise you won't know what RSA key it includes (so you won't know the capability itself).

This is certainly an "advanced" feature. If poor choices are made specifying the RSA key then certain significant features of Tahoe will be compromised (eg, if you re-use a key expecting to get a different object, you will be sorely disappointed). Still, for advanced uses, it is very important.

[exarkun]

And the "compliance test" issue is ticket:3961...

[exarkun]

​https://github.com/tahoe-lafs/tahoe-lafs/pull/1245

[exarkun]

Merged in ​https://github.com/tahoe-lafs/tahoe-lafs/commit/7b768fdaca6404285776225ecf1f69af626fd053