Issuer caching

[langsamu]

Similar to #30, this change introduces caching of IdPs (OAuth Authorization Servers).

First, the previous GetIssuerCallback, a lambda signature type, is converted to the interface IssuerProvider to make it easier to add state to its instances.

Then a CachingIssuerProvider is introduced to overlay atop an existing provider. It is used in the demo page:

reactive-authentication/index.html

Lines 23 to 27 in 05b292f

	const issuerUi = document.querySelector("idp-picker")
	const issuerProvider = new CachingIssuerProvider(issuerUi)
	const callbackUri = new URL("/callback.html", location.href).toString()
	const dPoPTokenProvider = new DPoPTokenProvider(callbackUri, ui.getCode.bind(ui), issuerProvider)

The cache is again internal to the caching provider instead of being taken from the caller. I will address this across the board later.

[langsamu]

@copilot fix merge conflicts in this pull request.

[Copilot]

@copilot fix merge conflicts in this pull request.

Resolved in commit 8d7b7ec by merging latest main into this branch and fixing the conflict in src/DPoPTokenProvider.ts.