Default SAML provider doesn't support the username picker

This issue has been migrated from [#10172](https://github.com/matrix-org/synapse/issues/10172).

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**Description:**

The default SAML mapping provider requires `mxid_source_attribute` to be specified but there is no usable attribute on our saml provider for this. The username picker is designed to solve this problem but it would require writing my own mapping provider. Could we have support in the default mapping provider for this functionality like there is with the openid connect one.