Fix n+1 query in extension registry list endpoint.

Created by: efritz

This alert keeps flapping: https://sourcegraph.app.opsgenie.com/alert/detail/78d8cf4f-2394-4df0-98f7-de97a44ed1f2-1582579884047/details.

It may be because the extension registry list endpoint will perform a SELECT of all ~230 extensions and perform an additional multi-join SQL query for each one in sequence before constructing a response. This will select the extensions, then do a single SQL query to get the latest release (making the endpoint only make 2 SQL queries rather than >230).

Created by: codecov[bot]

Codecov Report

Merging #8583 into master will decrease coverage by 0.01%. The diff coverage is 29.41%.

@@            Coverage Diff             @@
##           master    #8583      +/-   ##
==========================================
- Coverage   40.64%   40.62%   -0.02%     
==========================================
  Files        1305     1305              
  Lines       70032    70111      +79     
  Branches     6491     6491              
==========================================
+ Hits        28463    28486      +23     
- Misses      38954    39004      +50     
- Partials     2615     2621       +6

Impacted Files	Coverage Δ
...erprise/cmd/frontend/internal/registry/http_api.go	5.55% <0%> (-0.57%)
.../internal/registry/extension_connection_graphql.go	58.44% <0%> (-2.37%)
...md/frontend/internal/registry/extension_graphql.go	1.96% <0%> (-0.48%)
...d/frontend/internal/registry/extension_manifest.go	35.29% <28.94%> (-20.59%)
...rise/cmd/frontend/internal/registry/releases_db.go	79.01% <70.58%> (-6.1%)

Created by: lguychard

Just dropping by to say thanks!

Created by: ryanslade

Oh, wow nice. I think you just fixed something that was on my list: https://gitlab.sgdev.org/root/sourcegraph/-/issues/7544

Created by: tsenart

This has woken me up at improper hours too many times. It was on our list to fix (#7544), thank you for picking this up impromptu!