OAuth2: add JWT Profile fields from prometheus/common v0.67.3+

\`commoncfg.OAuth2\` gained JWT Profile support (RFC 7523) in prometheus/common v0.67.3. Both the v0mimir and v1 OAuth2 structs are missing these fields.

**New fields in \`commoncfg.OAuth2\`:**
- \`client_certificate_key_id\` (string)
- \`client_certificate_key\` (**Secret**)
- \`grant_type\` (string)
- \`signature_algorithm\` (string)
- \`iss\` (string)
- \`audience\` (string)
- \`claims\` (map)

**v0mimir** (`http/v0mimir/`):
1. Add fields to `OAuth2` struct in `http_client.go`
2. Update `toCommonOAuth2` / `fromCommonOAuth2` in `compat.go`
3. Update `V0HttpConfigOption` schema — mark `client_certificate_key` as `Secure: true`

**v1** (`http/`):
1. Add fields to `OAuth2Config` in `oauth2.go`
2. Update `NewOAuth2TokenSource` to pass JWT Profile fields to the token source
3. Update schema — mark `client_certificate_key` as `Secure: true`