Graceful failure (log) with SSO Connector loading

[jentfoo]

This PR fixes https://github.com/gravitational/security-findings/issues/47

This was originally attempted to be fixed in #34896, however this PR reverts those changes. The validation logic prior to #34896 was correct, and we should not attempt to fix this issue by making validation more lenient.

Instead a second commit provides an alternative fix where the connectors are being looped over and unmarshaled. Errors at that point will be logged rather than returned.

changelog: Fixed bug where configuration errors with an individual SSO connector impacted other connectors.

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[lxea]

This looks good to me, I think it would also be good to remove the Validate from Marshall and Unmarshall
Here https://github.com/gravitational/teleport/blob/master/lib/services/saml.go#L306-L308
and here: https://github.com/gravitational/teleport/blob/master/lib/services/saml.go#L328-L330
All the callers where it makes sense to validate do so manually already, and it doesnt seem right to have a http request in a marshaller

[codingllama]

A few minor comments.

[public-teleport-github-review-bot]

@jentfoo See the table below for backport results.

Branch	Result
branch/v12	Failed
branch/v13	Failed
branch/v14	Failed