Adds `DesktopSharedDirectoryStart` audit event

[ibeckermayer]

Overview

A shared directory is initialized by the TDP client generating a DirectoryID and sending a SharedDirectoryAnnounce followed by the TDP server sending back a SharedDirectoryAcknowledge

type SharedDirectoryAnnounce struct {
	DirectoryID uint32
	Name        string
}

type SharedDirectoryAcknowledge struct {
	ErrCode     uint32
	DirectoryID uint32
}

SharedDirectoryAnnounce is the only TDP message which sends us the name of the directory being shared (Name), after which the shared directory is always identified by its DirectoryID in other TDP message types.

As a matter of convenience, we want to show the shared directory's name in the audit event logs, which means that we need to keep an internal cache mapping DirectoryIDs to directory names. This is why I've added a sharedDirectoryNameMap to the WindowsService

https://github.com/gravitational/teleport/blob/200f16c471805262fa4a64fc3941a70334ca1dd3/lib/srv/desktop/windows_server.go#L96-L147

(Because we currently only allow for the sharing of a single directory, the sharedDirectoryNameMap currently maps sessionID to a directory's name. In the future, with multi-directory sharing enabled, we'd need a hash of the sessionID + DirectoryID per name).

A DesktopSharedDirectoryStart audit event is emitted via:

1. We receive an incoming SharedDirectoryAnnounce
2. We add an entry with that directory's name to the sharedDirectoryNameMap
3. Later we receive a SharedDirectoryAcknowledge
4. We grab the directory's name from the sharedDirectoryNameMap and use it to emit a DesktopSharedDirectoryStart audit event.

For discussion:

One point of note is that as currently designed, its technically possible that a SharedDirectoryAnnounce event is sent, and the server never responds, thus no audit event ever shows up. We could ameliorate this by adding a separate DesktopSharedDirectoryAnnounce (DesktopSharedDirectoryAttempt?) event which is emitted upon that initial announcement.

I think we should add it, because the fact that a user attempted to share a directory is conceivably relevant to a security team.

Other changes

The other changes are primarily a refactoring of the tests (as well as adding a couple of tests for this event), see the commit message for 200f16c471805262fa4a64fc3941a70334ca1dd3