feat: add indexer to metadata

[Sekhmet]

With support for multiple indexers in single Checkpoint instance we have to accomodate for those in our database.
This commit makes necessary changes in our metadata store.

Currently checkpoint still supports single indexer, but now metadata can accomodate it.

Test plan

1. Run checkpoint-template with this PR (you need to call resetMetadata before reset).
2. Run this query.
3. All looks good (we don't expose indexer in metadata on GraphQL yet, next PR).

{
  posts {
    id
    content
    tag
    tx_hash
    created_at_block
  }
  _metadatas {
    id
    value
  }
}

[bonustrack]

tACK works well