Add optional OIDC bearer auth on remote/virtual config writes (#136)

Remote and virtual configuration writes are unauthenticated, so anything that can reach the API can repoint or delete repos. Vault identity OIDC tokens give callers a verifiable identity.

- add `ARTIFACTAPI_OIDC_ISSUER`/`ARTIFACTAPI_OIDC_AUDIENCE`; unset issuer keeps auth off
- require a JWKS-verified bearer token on POST/PUT/DELETE of `/api/v2/remotes` and `/api/v2/virtuals`, 401 otherwise
- leave uploads, object deletes, probe, reads and `/v2` open
- discover the issuer lazily with a 10s client timeout; fail fast with 503 for 15s after a failed discovery
- log the verified `sub`; allow `Authorization` in CORS

Reviewed-on: #136
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

go.sum

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@@ -67,6 +69,8 @@ github.com/felixge/httpsnoop v1.1.0 h1:3YtUj32ZZkqZtt3sZZsClsymw/QDuVfpNhoA31zeO
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@@ -225,6 +229,8 @@ golang.org/x/exp v0.0.0-20231006140011-7918f672742d h1:jtJma62tbqLibJ5sFQz8bKtEM
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