fix(api): Protects DisableConfigDir from concurrent access using mutex

[yyoshiki41]

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model.ConfigPath is a global variable, so data races can happen if call api.DisableConfigDir() parallely.
see more: https://go.dev/doc/articles/race_detector#Primitive_unprotected_variable

reproduction

go test -race -run DisableConfigDir ./pkg/api/...
==================
WARNING: DATA RACE
Write at 0x000102907350 by goroutine 16:
  github.com/pdfcpu/pdfcpu/pkg/api.DisableConfigDir()
      /Users/yoshiki.nakagawa/repos/src/github.com/pdfcpu/pdfcpu/pkg/api/api.go:180 +0x64
  github.com/pdfcpu/pdfcpu/pkg/api.TestDisableConfigDir_Parallel.func1()
      /Users/yoshiki.nakagawa/repos/src/github.com/pdfcpu/pdfcpu/pkg/api/api_test.go:27 +0x58

Previous write at 0x000102907350 by goroutine 6:
  github.com/pdfcpu/pdfcpu/pkg/api.DisableConfigDir()
      /Users/yoshiki.nakagawa/repos/src/github.com/pdfcpu/pdfcpu/pkg/api/api.go:180 +0x38
  github.com/pdfcpu/pdfcpu/pkg/api.TestDisableConfigDir()
      /Users/yoshiki.nakagawa/repos/src/github.com/pdfcpu/pdfcpu/pkg/api/api_test.go:12 +0x2c
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.20.3/libexec/src/testing/testing.go:1576 +0x188
  testing.(*T).Run.func1()
      /opt/homebrew/Cellar/go/1.20.3/libexec/src/testing/testing.go:1629 +0x40

Goroutine 16 (running) created at:
  github.com/pdfcpu/pdfcpu/pkg/api.TestDisableConfigDir_Parallel()
      /Users/yoshiki.nakagawa/repos/src/github.com/pdfcpu/pdfcpu/pkg/api/api_test.go:25 +0x58
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.20.3/libexec/src/testing/testing.go:1576 +0x188
  testing.(*T).Run.func1()
      /opt/homebrew/Cellar/go/1.20.3/libexec/src/testing/testing.go:1629 +0x40

Goroutine 6 (finished) created at:
  testing.(*T).Run()
      /opt/homebrew/Cellar/go/1.20.3/libexec/src/testing/testing.go:1629 +0x5e4
  testing.runTests.func1()
      /opt/homebrew/Cellar/go/1.20.3/libexec/src/testing/testing.go:2036 +0x80
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.20.3/libexec/src/testing/testing.go:1576 +0x188
  testing.runTests()
      /opt/homebrew/Cellar/go/1.20.3/libexec/src/testing/testing.go:2034 +0x700
  testing.(*M).Run()
      /opt/homebrew/Cellar/go/1.20.3/libexec/src/testing/testing.go:1906 +0x950
  main.main()
      _testmain.go:49 +0x300
==================
--- FAIL: TestDisableConfigDir_Parallel (0.00s)
    api_test.go:31: DisableConfigDir is passed
    testing.go:1446: race detected during execution of test
FAIL
FAIL	github.com/pdfcpu/pdfcpu/pkg/api	0.370s
ok  	github.com/pdfcpu/pdfcpu/pkg/api/test	2.794s [no tests to run]
FAIL

⚠️ This is not a perfect solution as global variables can be changed from anywhere.
Unfortunately, I haven't solution protects this settings without breaking changes.

5. Fixes # ?

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[hhrutter]

Hello!

I wish we could have had a chance to discuss this before you open a PR.
Please tell us your detailed usecase causing the issue.
Thank you!

[hhrutter]

This is merged in finally!
Sorry it took so long.