Define Codespaces database table

Use a broad composite key to ensure uniqueness and stability over time.
Candidate "id" fields from Codespaces API not used as minimally documented
by GitHub and thus untrustworthy. Large composite primary key does have
performance penalities, but for the expected data volume this is not
presumed to be significant.

metrics/timescaledb/tables.py

@@ -4,6 +4,16 @@
 metadata = MetaData()
 
 
+GitHubCodespaces = Table(
+    "github_codespaces",
+    metadata,
+    Column("created_at", TIMESTAMP(timezone=True), primary_key=True),
+    Column("organisation", Text, primary_key=True),
+    Column("repo", Text, primary_key=True),
+    Column("user", Text, primary_key=True),
+    Column("last_used_at", TIMESTAMP(timezone=True)),
+)
+
 GitHubRepos = Table(
     "github_repos",
     metadata,