added cache to discovery and instropection

[coveralls]

Pull Request Test Coverage Report for Build 151

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.05%) to 96.35%

---

Files with Coverage Reduction	New Missed Lines	%
./kong/plugins/oidc/handler.lua	1	98.11%
<!--	Total:	1		-->

Totals
Change from base Build 134:	0.05%
Covered Lines:	132
Relevant Lines:	137

---

💛 - Coveralls