azapi: Support client_certificate field

[ms-henglu]

related issue: #488

This PR also refactors the logics that build credentials and adds test cases for different authentication methods.

Test results:

1. az cli

=== RUN   TestAccAuth_azcli
=== PAUSE TestAccAuth_azcli
=== CONT  TestAccAuth_azcli
--- PASS: TestAccAuth_azcli (39.17s)
PASS

2. sp client secret
   running test....

3. sp client certificate path

=== RUN   TestAccAuth_clientCertificatePath
=== PAUSE TestAccAuth_clientCertificatePath
=== CONT  TestAccAuth_clientCertificatePath
--- PASS: TestAccAuth_clientCertificatePath (38.29s)
PASS

4. sp client certificate

=== RUN   TestAccAuth_clientCertificate
=== PAUSE TestAccAuth_clientCertificate
=== CONT  TestAccAuth_clientCertificate
--- PASS: TestAccAuth_clientCertificate (38.17s)
PASS

5. oidc

6. msi

adminuser@henglu-vm:~/terraform-provider-azapi$ TF_ACC=1 /snap/bin/go test -v ./internal/services -run TestAccActionDataSource_providerAction -timeout 10m -ldflags="-X=github.com/Azure/terraform-provider-azapi/version.ProviderVersion=acc"

=== RUN   TestAccActionDataSource_providerAction
=== PAUSE TestAccActionDataSource_providerAction
=== CONT  TestAccActionDataSource_providerAction
--- PASS: TestAccActionDataSource_providerAction (32.21s)
PASS
ok  	github.com/Azure/terraform-provider-azapi/internal/services	32.218s