add feature for check custom interfaces described in ruleguard rules

[peakle]

add feature for check local described interfaces in rules or in the same repo

[quasilyte]

I would start from allowing inline interfaces:

Type.Implements(<inline interface>)

Like
Type.Implements(`interface{String() string}`)

For more complex cases one could use a constant:

const dbInterface = `interface{
  Query(query string, args ...interface{}) (*sql.Rows, error)
  Close() error
}`

Note that types like sql.Rows will be resolved in the context of the current rule. This will make it possible to reference types outside of the stdlib by using dsl.Import before using Type.Implements on the string that refers to these external types.