Allow client to access the repo's terms table.

This makes a TermIndex instance available on the client
and modified TermSqlIndex to allow foreign wiki access.

Note that this new functionality isn't used yet, PropertyParserFunction
will be modified to use it in a follow-up change.

Bug: 46363
Change-Id: I9a822bee7af493c4a7371b712dac369a8191daee
Merge "Allow client to access the repo's terms table."