fix: project_api_key data source missing project_assignment attribute

[andreaangiolillo]

Description

Ticket: INTMDB-969

This PR adds the project_assignment to the data source project_api_key has we forgot to add it

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

Tested locally + Added acceptance tests

terraform state show data.mongodbatlas_project_api_key.test
# data.mongodbatlas_project_api_key.test:
data "mongodbatlas_project_api_key" "test" {
    api_key_id         = "64c7eb2c175e1e5d357fb05c"
    description        = "TF key"
    id                 = "terraform-20230731171109811200000001"
    private_key        = "********-****-****-56f6fb43a7b3"
    project_assignment = [
        {
            project_id = "63f4d4a47baeac59406dc131"
            role_names = [
                "GROUP_OWNER",
                "GROUP_READ_ONLY",
            ]
        },
        {
            project_id = "64b8d3219c4cdc3f9108e318"
            role_names = [
                "GROUP_READ_ONLY",
            ]
        },
    ]
    project_id         = "63f4d4a47baeac59406dc131"
    public_key         = "estakgih"
    role_names         = [
        "GROUP_OWNER",
        "GROUP_READ_ONLY",
    ]
}

terraform state show data.mongodbatlas_project_api_keys.test
# data.mongodbatlas_project_api_keys.test:
data "mongodbatlas_project_api_keys" "test" {
    id         = "terraform-20230731171830248700000001"
    project_id = "63f4d4a47baeac59406dc131"
    results    = [
        {
            api_key_id         = "64c7eb2c175e1e5d357fb05c"
            description        = "TF key"
            private_key        = "********-****-****-56f6fb43a7b3"
            project_assignment = [
                {
                    project_id = "63f4d4a47baeac59406dc131"
                    role_names = [
                        "GROUP_OWNER",
                        "GROUP_READ_ONLY",
                    ]
                },
                {
                    project_id = "64b8d3219c4cdc3f9108e318"
                    role_names = [
                        "GROUP_READ_ONLY",
                    ]
                },
            ]
            public_key         = "estakgih"
            role_names         = [
                "GROUP_OWNER",
                "GROUP_READ_ONLY",
            ]
        },
    ]
}

[Zuhairahmed]

LGTM