Fix authorization credentials field to use password input type

[dengjulie]

Summary

• The "Authorization Header - Credentials" field in the OnCall and webhook receiver schemas was rendered as a plain text input instead of a password input, exposing the secret value on screen.
• Change InputType from Text to Password in both schemas to match the field's Secure: true designation, consistent with how other secure fields are rendered.

Fixes grafana/grafana#129717

Test plan

• go test ./receivers/oncall/... passes
• go test ./receivers/webhook/... passes

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