make use of conformance testing instead of actual tokens

[loosebazooka]

Examples fails a lot on forked PRs, but we can just use the conformance token instead.

• introduce ability to read from SIGTSORE_JAVA_ID_TOKEN
• set SIGSTORE_JAVA_ID_TOKEN in examples (requires another release before fully working)

Unit tests use conformance tokens

• keep a few small tests for web/github oidc tokens
• use conformance token whenever possible via TokenStringOidcClient