Support subscribe PR of specially author

[liangyuanpeng]

Is your feature request related to a problem? Please describe.
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Describe the solution you'd like

I want to subscribe myself PR, so maybe the command like this:

/github subscribe owner/repo pulls:users/liangyuanpeng or /github subscribe owner/repo pulls:{actor:"liangyuanpeng"}

Thanks you!

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[baishalighosh]

Hi team, any chance of getting this prioritized?