encryption: Lazily decrypt `user_external_accounts`

[efritz]

Stacked on #40282. This PR wraps the user_external_accounts.auth_data and user_external_accounts.account_data fields in an encryptable struct.

Test plan

main-dry-run: https://buildkite.com/sourcegraph/sourcegraph/builds/166839

[sourcegraph-bot]

Codenotify: Notifying subscribers in CODENOTIFY files for diff f470b78abf6bac78dd56c533026551c314e1a630...19f0e3c0887f3841bfc6dd501ade19b4ed6ab2f5.

Notify	File(s)
@eseliger	internal/database/external_accounts.go internal/database/external_accounts_test.go internal/extsvc/data.go internal/extsvc/gerrit/account.go internal/extsvc/github/common.go internal/extsvc/gitlab/user.go internal/extsvc/perforce/account.go internal/extsvc/types.go
@indradhanush	enterprise/cmd/repo-updater/internal/authz/integration_test.go enterprise/cmd/repo-updater/internal/authz/perms_syncer.go enterprise/cmd/repo-updater/internal/authz/perms_syncer_test.go
@unknwon	enterprise/cmd/frontend/internal/auth/githuboauth/session.go enterprise/cmd/frontend/internal/auth/gitlaboauth/session.go enterprise/cmd/frontend/internal/auth/openidconnect/user.go enterprise/cmd/frontend/internal/auth/saml/user.go enterprise/cmd/repo-updater/internal/authz/integration_test.go enterprise/cmd/repo-updater/internal/authz/perms_syncer.go enterprise/cmd/repo-updater/internal/authz/perms_syncer_test.go enterprise/internal/authz/bitbucketserver/provider.go enterprise/internal/authz/bitbucketserver/provider_test.go enterprise/internal/authz/gerrit/gerrit.go enterprise/internal/authz/github/github.go enterprise/internal/authz/github/github_test.go enterprise/internal/authz/gitlab/common_test.go enterprise/internal/authz/gitlab/oauth.go enterprise/internal/authz/gitlab/oauth_test.go enterprise/internal/authz/gitlab/sudo.go enterprise/internal/authz/gitlab/sudo_test.go enterprise/internal/authz/perforce/perforce.go enterprise/internal/authz/perforce/perforce_test.go

[unknwon]

Nice!