Syntactic indexing: add database schema and dbworker

[keynmol]

Closes #59794

This PR:

• Adds a basic schema for syntactic code intel indexes (up and down migration)
• Sets up everything required to use dbworker interface
• Adds a stub indexing worker implementation
• Adds tests to ensure conformance with dbworker interface (run against real postgres)

Test plan

New tests to ensure schema conformance.

For manual testing:

1. Start the configuration: sg start codeintel-syntactic
2. Execute this SQL query: insert into syntactic_scip_indexes(id, commit, repository_id, outfile) values (nextval('syntactic_scip_index_id_seq'), '8c0cc755be302107b675824b0fb1b985992092b8', 1, 'index.scip');
3. Observe the worker pick up the job:

[syntactic-c...1] INFO syntactic-code-intel-worker workerutil/worker.go:329 Dequeued record for processing {"TraceId": "a0b9c4a988d9e46ac42b8c4deb63d670", "SpanId": "6483b7e47fa617f2", "name": "syntactic_code_intel_indexer", "id": "1"}
[syntactic-c...1] INFO syntactic-code-intel-worker.Handle shared/indexing_worker.go:34 Stub indexing worker handling record {"TraceId": "a0b9c4a988d9e46ac42b8c4deb63d670", "SpanId": "470708143297b7ba", "handle": {"id": 1, "repository name": "github.com/sourcegraph-testing/zap", "commit": "8c0cc755be302107b675824b0fb1b985992092b8"}}

[keynmol]

As we now require the repo table to be populated for the syntactic_scip_indexes_with_repository_name view to work in joins, it's useful to have repo updater running locally.

[varungandhi-src]

Added some more comments; will do a third pass shortly over some of the SQL.

[varungandhi-src]

No blocking comments, but some questions and suggestions for improving readability.

[varungandhi-src]

Thanks for adding this comment, it's very useful

[keynmol]

@varungandhi-src once the build is green I will merge this - if you have any more comments, please add them here or raise a separate issue.