Int64Slice not support by read toml file

[Edelweiss-Snow]

Checklist

• Are you running the latest v2 release? The list of releases is here.
• Did you check the manual for your release? The v2 manual is here.
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What problem does this solve?

I used toml file input configuration, toml parsing int variables will become int64, but this file altsrc/map_input_source.go does not support int64Slice，toml Unmarshal number (example:1,2,3) will become int64 (1,2,3), so i nedd it

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• the terminal output for a particular error case is confusing, and
  I think it could be improved

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[dearchap]

PR #1551