Add config for introspection_endpoint_auth_method

[tommilligan]

It was not possible to set the auth method for introspection through the config.

This was found when trying to use token introspection with ORY Hydra, which requires Basic or OAuth authorisation for introspection.

This PR:

• adds introspection_endpoint_auth_method as required by lua-resty-openidc
• adds this value to config, schema, readme
• adds unit test
• does not add a default value for backwards compatibility (will remain as nil)

[coveralls]

Pull Request Test Coverage Report for Build 90

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 95.968%

---

Totals
Change from base Build 89:	0.03%
Covered Lines:	119
Relevant Lines:	124

---

💛 - Coveralls

[tommilligan]

@Trojan295 are you interested in help maintaining this project? I'm currently working on features to match Kong's enterprise OAuth Introspection plugin

[tommilligan]

ping @Trojan295

@zandbelt is this of interest to you as well?

[zandbelt]

@tommilligan I'm interested but I don't have the bandwidth to contribute.

[Trojan295]

@tommilligan, sorry for the late response, but I'm out of this project a bit.

Still, it looks OK from my side.

@phirvone @tsyrjanen , can you try to prepare a new release for LuaRocks? There were a few PR accepted since the last release.

[tommilligan]

@Trojan295 thanks, no worries at all