fix(types): Add AttachmentType and use for envelope attachment_type property

[timfish]

Attachment.attachmentType was changed to use a string union. The attachment_type header property should match so I added an extra type for this.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	77.47 KB (+0.11% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	68.66 KB (+0.11% 🔺)
@sentry/browser (incl. Tracing, Replay with Canvas) - Webpack (gzipped)	72.58 KB (+0.11% 🔺)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	62.22 KB (+0.13% 🔺)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	32.91 KB (+0.22% 🔺)
@sentry/browser (incl. browserTracingIntegration) - Webpack (gzipped)	32.91 KB (+0.22% 🔺)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	31.36 KB (+0.06% 🔺)
@sentry/browser (incl. sendFeedback) - Webpack (gzipped)	31.37 KB (+0.06% 🔺)
@sentry/browser - Webpack (gzipped)	22.57 KB (+0.06% 🔺)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	75.54 KB (+0.14% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	67.13 KB (+0.12% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	33 KB (+0.27% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	23.98 KB (+0.04% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	210.76 KB (+0.11% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	99.57 KB (+0.23% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	71.67 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	36.16 KB (+0.29% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	68.94 KB (+0.11% 🔺)
@sentry/react - Webpack (gzipped)	22.6 KB (+0.06% 🔺)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	85.36 KB (+0.08% 🔺)
@sentry/nextjs Client - Webpack (gzipped)	49.77 KB (+0.15% 🔺)
@sentry-internal/feedback - Webpack (gzipped)	17.41 KB (0%)