Fix local client

[1d4n]

Summary

Fixed the file methods of the local client and added an option to choose a custom directory for it.

Details and comments

Example of initializing a local service with a custom directory:

from qiskit_ibm_experiment import IBMExperimentService
service = IBMExperimentService(local=True, local_db_dir='/home/usr/results')

Fixes #
There were problems with loading and saving .zip files.
E.g. this code raised the exception: TypeError: write() argument must be str, not _io.BytesIO:

from qiskit_experiments.library.characterization import T1
import numpy as np
from qiskit_aer import AerSimulator
backend = AerSimulator()
t1_delays = np.arange(1e-6, 600e-6, 50e-6)
exp = T1(physical_qubits=(0,), delays=t1_delays)
exp_data = exp.run(backend=backend).block_for_results()

from qiskit_ibm_experiment import IBMExperimentService
exp_service = IBMExperimentService(local=True)
exp_data.service = exp_service
exp_data.save() # this line caused the exception (it calls to IBMExperimentService.file_upload() and then to LocalExperimentClient.save()).

• In order to read a zip file, we need to open it in 'rb' mode.
• In LocalExperimentClient.experiment_file_download(): if we use io.BytesIO and read the data, we need to use file.seek(0) so next time we read the data, it will start reading from the beginninng.

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