Add go-sumtype

[AlekSi]

https://github.com/haya14busa/gosum
I found it very useful to ensure that a new enum value is handled everywhere.

[AlekSi]

Another alternative: https://github.com/BurntSushi/go-sumtype

[jirfag]

thank you for suggestion

[matoous]

The first proposed linter is unmaintained, if we decide to add this linter I suggest going with the second one.

[SVilgelm]

it is not possible to use go-sumtype as a library

[cristaloleg]

Sounds like https://github.com/nishanths/exhaustive which is already supported.

[AlekSi]

I wasn't able to make exhaustive linter work with types like go-sumtype there: https://github.com/percona/pmm/blob/PMM-2.0/api/agentpb/agent.go