feat(types): export Plugin type

[oktapodia]

Description

It seems that the Plugin type is not exported, it would be useful so we can type our custom plugins

Motivation and Context

Being able to correctly type our plugins

How has this been tested?

Used locally a forked version

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[coveralls]

coverage: 97.236%. remained the same
when pulling 935195b on oktapodia:patch-1
into c18a580 on chimurai:master.

[chimurai]

Thanks!