Use gofumpt if available, and enable gofumpt linter

[thaJeztah]

• follow-up to format code with gofumpt #3794
• relates to golangci-lint: enable more linters #3770

gofumpt (https://github.com/mvdan/gofumpt) provides a supserset of gofmt / go fmt, but not every developer may have
it installed, so for situations where it's not available, fall back to gofmt.

As our code has been formatted with gofumpt already, in most cases contributions
will follow those formatting rules, but in some cases there may be a difference,
which would already be flagged by manual code review, but let's also enable the
gofumpt linter.

With this change, make fmt will use gofumpt is available; gofumpt has been
added to the dev-container, so make -f docker.Makefile fmt will always use it.

[thaJeztah]

@crazy-max@vvoland ptal 😄

[thaJeztah]

gofumpt automatically excludes vendor/ so no need to use go list

[thaJeztah]

Hm... interesting; somehow my gofumpt didn't format all files?

#17 87.07 e2e/plugin/basic/basic.go:17: File is not `gofumpt`-ed (gofumpt)
#17 87.07 if err := os.MkdirAll(p, 0755); err != nil {
#17 87.07 cli/context/store/metadatastore.go:31: File is not `gofumpt`-ed (gofumpt)
#17 87.07 if err := os.MkdirAll(contextDir, 0755); err != nil {
#17 87.07 cli/context/store/metadatastore.go:38: File is not `gofumpt`-ed (gofumpt)
#17 87.07 return os.WriteFile(filepath.Join(contextDir, metaFile), bytes, 0644)
#17 87.07 cli/context/store/store.go:249: File is not `gofumpt`-ed (gofumpt)

[codecov-commenter]

Codecov Report

Merging #3798 (c2f1671) into master (65438e0) will not change coverage.
The diff coverage is 66.66%.

Additional details and impacted files

@@ Coverage Diff @@## master #3798 +/- ##
=======================================
Coverage 59.21% 59.21% =======================================
Files 288 288 Lines 24605 24605 =======================================
Hits 14571 14571 Misses 9159 9159 Partials 875 875 

[thaJeztah]

Looks like I need to add an explicit -lang=1.19, otherwise it doesn't pick up the go version for which to format. Probably because we don't have a go.mod

[thaJeztah]

@crazy-max ptal 🤗

[crazy-max]

LGTM if opt-in.

Do we want a scheduled worklow to fix-up and open PR as semi-automation process?

[thaJeztah]

Let me bring this one in 👍