Add Set-AzBootstrapConfig and Get-AzBootstrapConfig public functions

[Copilot]

This PR adds two new public functions to help manage the .az-bootstrap.jsonc configuration file:

Set-AzBootstrapConfig

This helper function allows users to easily add or update template aliases in the configuration file:

# Add a new template alias
Set-AzBootstrapConfig -TemplateAlias "terraform" -Value "https://github.com/kewalaka/terraform-azure-starter-template"

# The same command can be used to update an existing alias
Set-AzBootstrapConfig -TemplateAlias "terraform" -Value "https://github.com/my-org/terraform-template"

Get-AzBootstrapConfig

This function displays the existing configuration including template aliases and default location:

Get-AzBootstrapConfig

Example output:

[az-bootstrap] Configuration file path: C:\Users\username\.az-bootstrap.jsonc

[az-bootstrap] Template Aliases:
  terraform -> https://github.com/kewalaka/terraform-azure-starter-template
  bicep -> https://github.com/kewalaka/bicep-azure-starter-template

[az-bootstrap] Default Location: eastus

The function also returns the configuration object for use in pipelines or scripts.

Implementation Details

• Both functions follow established patterns in the module
• Full test coverage with Pester tests
• Properly handles JSON comments in the configuration file
• Creates the configuration file if it doesn't exist
• Shows configuration file path for easier manual editing

These new functions provide users with a more convenient way to manage their default template repositories without directly editing the configuration file.

Fixes #44.

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