feat(react-router): Update loadContext type to be compatible with middleware

[chargome]

Will create follow-up PR that tests a middleware e2e setup

closes #17749

[github-actions]

node-overhead report 🧳

Note: This is a synthetic benchmark with a minimal express app and does not necessarily reflect the real-world performance impact in an application.

Scenario	Requests/s	% of Baseline	Prev. Requests/s	Change %
GET Baseline	8,696	-	8,683	+0%
GET With Sentry	1,401	16%	1,377	+2%
GET With Sentry (error only)	5,983	69%	5,879	+2%
POST Baseline	1,203	-	1,205	-0%
POST With Sentry	528	44%	516	+2%
POST With Sentry (error only)	1,059	88%	1,048	+1%
MYSQL Baseline	3,305	-	3,270	+1%
MYSQL With Sentry	457	14%	464	-2%
MYSQL With Sentry (error only)	2,670	81%	2,676	-0%

View base workflow run

[wong2]

Thanks!