Introduce database import and export protocol messages

[farost]

Release notes: usage and product changes

Add migration protocol messages for usage in database import and database export operations:

• a unidirectional database_export stream from a TypeDB server to export a specific database, similar to TypeDB 2.x;
• a bidirectional databases_import stream between a client and a server to import an exported 2.x/3.x TypeDB database into a TypeDB 3.x server from a client.

The format of migration items used for these operations is an extended version of TypeDB 2.x's migration items, so it is backward compatible with 2.x database files. Important: it's not intended to import 3.x databases into 2.x servers.

Implementation

Add Migration { Item } message. The format is an extended version of the 2.x protocol, so it contains "outdated" fields for its compatibility with old databases.

Add Migration { Export } message. This operation consists of a single client Req { database } and multiple streamed server responses:

1. An initial response with the schema.
2. An unlimited number of migration items (multiple messages with multiple items in one message for potential optimizations)
3. A Done message to signal that the server is ready to close the stream without errors. It can be substituted by a silent stream closure, but I preferred explicitness here.

Add Migration { Import } message. This operation consists of a stream of client requests:

1. An initial request with the name of the database and its schema string.
2. An unlimited number of migration items.
3. A Done message to signal that the client is finished without errors, and the server can perform the final validation. This Done message is required and cannot be removed because the client has to check whether there were finalization errors or not.

and a stream of server responses (actually, there is either a single Done or a single error, but the stream is needed to return errors at any stage of the communication).

[farost]

I've placed all migration-related messages in a single file, so it's "encapsulated". However, this introduces an additional layer of optional client and server in Rust while unpacking this message, so it's a little irritating. Not sure if it's worth it or not. I like this design more, I guess.

[farost]

I was planning to reuse this error, but I understood that I don't need non-terminal errors in my protocol. However, I think that it's reasonable to generalize error messages for the whole TypeDB protocol.