Add mapping for encryption key ID while doing IMPORT TABLESPACE

We need some mechanism to remap the encryption key IDs when importing a tablespace.

It may be simplest to make IMPORT TABLESPACE to look for a new file, tablename.enc (or similar), which would contain a mapping from old encryption key IDs (and key versions) to the correspond ones in the importing server.

Typical use case is along the following lines:

1. Export encrypted table(s) from backup on source. Let's assume they are encrypted with default key or key #M on source
2. Add source encryption key to target encryption key list as entry #N (N != M in a general case)
3. Create empty tables on the target with encryption_id_key = N (the sources key)
4. Discard the tablespaces of the empty tables on the target
5. Import the tablespaces from the source backup to the target, mapping or replacing the key IDs/versions to use proper keys now on target