Lists all available biomart datasets for a given mart.
Each mart contains multiple datasets. This function returns all datasets
available in the specified mart as a CSV string.
Args:
mart (str): The mart identifier to list datasets from.
Valid values include: ENSEMBL_MART_ENSEMBL, ENSEMBL_MART_MOUSE,
ENSEMBL_MART_ONTOLOGY, ENSEMBL_MART_GENOMIC, ENSEMBL_MART_SNP,
ENSEMBL_MART_FUNCGEN
Returns:
str: CSV-formatted table of all datasets with their display names and descriptions.
Example:
list_datasets("ENSEMBL_MART_ENSEMBL")
>>> "name,display_name,description
hsapiens_gene_ensembl,Human genes,Human genes (GRCh38.p13)
mmusculus_gene_ensembl,Mouse genes,Mouse genes (GRCm39)
..."