fix: yaml output of servers userdata is bytearray instead of base64

[Benjosh95]

Description

Problem:
The commands for server list and describe returns the userData as byteArray instead of base64 string.

Jira-Tickets:

• https://jira.schwarz/browse/STACKITSDK-139
• https://jira.schwarz/browse/STACKITCLI-219

Testing instruction:

1. Create a server with userData ("test") via TF config looking something like this:

            resource "stackit_server" "server-01" {
              project_id = var.project_id
              name       = "user-data-test-server"
              boot_volume = {
                source_type           = "image"
                size                  = 32
                source_id             = "a2c127b2-b1b5-4aee-986f-41cd11b41279" // Ubuntu 24.04 image
                delete_on_termination = true
              }
              machine_type = "t1.1"
              availability_zone  = "eu01-1"
              user_data = "test"
            `}`

2. Build the CLI for this branch
3. Run the two commands stackit server describe -o yaml and stackit server list -o yaml to see if userData is correctly output as base64

Checklist

• Issue was linked above
• Code format was applied: make fmt
• Examples were added / adjusted (see e.g. here)
• Docs are up-to-date: make generate-docs (will be checked by CI)
• Unit tests got implemented or updated
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)

[github-actions]

Merging this branch changes the coverage (2 decrease, 1 increase)

Impacted Packages	Coverage Δ	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/server/describe	36.52% (-0.32%)	👎
github.com/stackitcloud/stackit-cli/internal/cmd/server/list	33.78% (-0.46%)	👎
github.com/stackitcloud/stackit-cli/internal/pkg/utils	55.68% (+10.48%)	🎉

---

Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/stackitcloud/stackit-cli/internal/cmd/server/describe/describe.go	36.52% (-0.32%)	115 (+1)	42	73 (+1)	👎
github.com/stackitcloud/stackit-cli/internal/cmd/server/list/list.go	33.78% (-0.46%)	74 (+1)	25	49 (+1)	👎
github.com/stackitcloud/stackit-cli/internal/pkg/utils/utils.go	58.46% (+14.46%)	65 (+15)	38 (+16)	27 (-1)	🎉

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/stackitcloud/stackit-cli/internal/pkg/utils/utils_test.go