feat: Add retry logic with exponential backoff

[CaptainDriftwood]

Summary

Add configurable retry logic with exponential backoff for transient failures.

Motivation

• Network issues can cause temporary connection failures
• ICAP servers may be temporarily unavailable during updates/restarts
• Production environments need resilience against transient errors

Proposed Implementation

• Add retries and backoff_factor parameters to IcapClient
• Retry on IcapConnectionError and IcapTimeoutError
• Use exponential backoff: delay = backoff_factor * (2 ** attempt)
• Make retry behavior configurable (max retries, which exceptions to retry)

Example Usage

from pycap import IcapClient

# Retry up to 3 times with exponential backoff
client = IcapClient(
    'localhost',
    retries=3,
    backoff_factor=0.5  # 0.5s, 1s, 2s delays
)

with client:
    response = client.scan_file('/path/to/file.pdf')