[backport v2.12] [SURE-10842] Add config option to configure the number of routines when creating bundles in fleet apply

[rancherbot]

This is a backport issue for #4233, automatically created via GitHub Actions workflow initiated by @kkaempf

Original issue body:

It is fixed to 4, currently and the code creates an additional routine.

See

fleet/internal/cmd/cli/apply/apply.go

Line 108 in 02ac738

const bundleCreationMaxConcurrency = 4

We might need to configure this to allow more or less concurrent bundle creation.

Created after: SURE-10842

[kkaempf]

Waiting for product approval

[kkaempf]

Waiting for product approval

Approved.