Revision specs are a nice way for a SCMClient to convert user-provided

input to formal values that the SCMClient can use to determine

revisions. While most SCMClients use the same keys (base, tip, and a

few others), they have different needs when it comes to values (some use

strings, some integers, and in the case of TFS, lists or tuples of

values). On top of that, SOS needs its own special keys.
A previous attempt was made at creating a TypedDict that could

describe the possible keys, but this was limiting in types of values,

assuming they were all strings. TypedDict also has no support for

arbitrary keys.
This change does a few things:


Makes all values in TypedDict an object instead of a str. (Note

   that we don't use Any, as that bypasses any type checking when,

   say, reassigning the value, and we want consistency).


Adds a flexible extra key that can be used for things like SOS's

   changeset and selection stuff. (Clients can define their own

TypedDict for their needs and set it or cast as appropriate.)


Updates the function signatures and documentation for all

   implementations to be consistent with each other.


Fixes up some code affected by the type definitions, helping ensure

   we're putting the right types in the keys.


This will ensure we don't hit any issues going forward with assumptions

around keys, types, and values in usage of revision specs, and can make

informed decisions on when to support a new top-level key.

Unit tests pass.
Verified the type checker was happy in typed code using

SCMClientRevisionSpec (there's lots of other code using it that hasn't

yet been updated though).
Posted a change using this code.