Allow parameters to be omitted so they can be supplied at deploy-time

[shenglol]

Bicep version
196661b

Describe the bug
Currently, if a required parameter is not assigned a value in a .bicepparams file, a BCP258 error will be generated.

IMHO this should not be the expected behavior, since it is legit for a user to provide the value at deploy time via the Azure CLI / Azure PowerShell command prompt. The quick fix might be to change the error to a warning.

[alex-frankel]

Agree that we need to revisit this behavior given at least az CLI allows you to add parameters in the deployment command that are not present in the parameters file.

@stephaniezyen -- can we discuss this in the next bicep params sync?

[ggirard07]

+1 on this... I wanted to migrate from json parameter file but this is currently blocking me as I cannot omit secret parameters I want to seed in key vaults. With json parameter file, I could not specify those and set them from CLI instead.

[alex-frankel]

Closing as dup of #12019