Update secret handling and configuration in Bicep files

[FrodeHus]

Update secret handling and configuration in Bicep files

Updated BeyondTrustCredentialClient to retrieve secret name from
KEYVAULT_SECRET environment variable and handle deserialization
errors. Added keyvaultSecretName property to functionAppConfig
type in main.bicep and updated related modules to use this
property. Updated functionConfig parameter to include
keyvaultSecretName. Added keyvaultSecretName parameter to
functionapp.bicep and secretName parameter to
vault-role-assignment.bicep. Changed roleAssignment scope to
specific secret within Key Vault.

PR Classification

Code enhancement to improve configuration management and error handling.

PR Summary

Updated the BeyondTrustCredentialClient to use an environment variable for the secret name and improved error handling. Modified Bicep files to support the new configuration.

• BeyondTrustCredentialClient.cs: Read secret name from KEYVAULT_SECRET environment variable and added null-coalescing operator for deserialization.
• main.bicep: Added keyvaultSecretName property to functionAppConfig and updated functionappModule and vaultSecretUserRoleAssignment modules.
• functionapp.bicep: Added keyvaultSecretName parameter and set KEYVAULT_SECRET environment variable.
• vault-role-assignment.bicep: Added secretName parameter, changed roleAssignment scope, and added keyVaultSecret resource.