Encode newlines in headers when using ByteGenerator

[sethmlarson]

Related:

• CVE-2024-6923
• email.policy.default - gotcha with re-using parsed headers with embedded newlines #121650

Linked PRs

• gh-144125: email: verify headers are sound in BytesGenerator #144126
• [3.10] gh-144125: email: verify headers are sound in BytesGenerator #144180
• [3.13] gh-144125: email: verify headers are sound in BytesGenerator #144181
• [3.14] gh-144125: email: verify headers are sound in BytesGenerator #144182
• [3.12] gh-144125: email: verify headers are sound in BytesGenerator #144188
• [3.11] gh-144125: email: verify headers are sound in BytesGenerator #144189