Add Profile as data source

[maveonair]

This pull request introduces a new data source for Incus profiles in the Terraform provider for Incus.

This makes it possible to use an existing profile that was not created by Terraform, such as the default profile as mentioned in #58:

data "incus_profile" "default" {
  name = "default"
}

resource "incus_instance" "d1" {
  profiles = [data.incus_profile.default.name]
  image    = "images:debian/12"
  name     = "d1"
}

Documentation:

• docs/data-sources/profile.md: Added documentation for the new incus_profile data source, including example usage, argument reference, and attribute reference.

Changes:

• internal/profile/datasource_profile.go: Created a new Go file datasource_profile.go that includes the ProfileDataSource struct and its associated methods. This struct represents the Incus profile data source, and the methods handle the metadata, schema, configuration, and reading of the data source.
• internal/provider/provider.go: Updated the DataSources method of the IncusProvider struct to include the new profile.NewProfileDataSource function, which creates a new instance of the ProfileDataSource.