feat: support template README content

[tgolob]

Summary

• add an optional readme argument to runpod.create_template()
• include and escape the README content in the saveTemplate GraphQL mutation
• add coverage for defaults and multiline/quoted README values

Closes #461.

Validation

• uv run --group test pytest -q --no-cov tests/test_api/test_mutations_templates.py — 3 passed
• uv run --group test pytest -q --no-cov tests/test_api/test_ctl_commands.py — 16 passed
• python3 -m py_compile runpod/api/mutations/templates.py runpod/api/ctl_commands.py
• git diff --check

The repository's default coverage threshold is not meaningful for a targeted test invocation, so the focused runs use --no-cov; the tests themselves pass.