Add simple repr for EnsemblDB

[ivirshup]

import genomic_features as gf

ensdb = gf.ensembl.annotation(species="Hsapiens", version="108")
ensdb
# EnsemblDB(organism='Homo sapiens', ensembl_release='108')

Open to changing this up.

This also adds a metadata field for reporting the metadata table:

ensdb.metadata
# {'Db type': 'EnsDb',
#  'Type of Gene ID': 'Ensembl Gene ID',
#  'Supporting package': 'ensembldb',
#  'Db created by': 'ensembldb package from Bioconductor',
#  'script_version': '0.3.7',
#  'Creation time': 'Fri Oct 28 05:24:43 2022',
#  'ensembl_version': '108',
#  'ensembl_host': 'localhost',
#  'Organism': 'Homo sapiens',
#  'taxonomy_id': '9606',
#  'genome_build': 'GRCh38',
#  'DBSCHEMAVERSION': '2.2'}