Allow a mongod started in embedded-router mode to initialize the router's sharding state such that:

• When requests for sharding-initialization commands (addShard, enableSharding, shardCollection) are received on the router port, the router role is able to successfully run these commands.

Note that this will require a few pieces of work:

• Mapping out through experimentation + confirmation with the sharding team what sharding-state needs to be initialized, and what needs to be separately registered between router and shard services (i.e. CatalogCacheLoader, Grid, ShardingState, and any others)
• Performing the dual registration and initialization of any decorations/globals that require separate implementations for shard and router, and removing any non-role-aware accesses
• Writing logic in sharding_initialization and sharding_initialization_mongod that is called from mongod main when embedded router is enabled, that sets up the required state for the router to run the sharding-initialization commands
• Integrating with a test-suite that we produce elsewhere for a single-shard cluster, or writing a noPassthrough test that starts a mongod with embedded router and confirms it can add a shard successfully.

Note that this will likely require multiple commits and separate JIRA tickets/subtasks should be cleaved off. But because I am not exactly sure of the best way to break down the work ahead-of-time, I am creating this ticket as a placeholder/box.