feat(tracing): Record transaction name source when name set directly

[lobsterkatie]

This adds transaction name source to transaction metadata in various situations in which the name is set directly: starting a manual transaction, assigning to the name property of a transaction, calling a transaction's setName method, and changing the name using beforeNavigate.

In order to distinguish manually-created transactions from automatically-created ones, all internal uses of startTransaction have been changed so that they are happening directly on the hub, rather than through the top-level Sentry.startTransaction() wrapper.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.36 KB (+0.11% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	59.89 KB (+0.05% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	17.95 KB (+0.09% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	52.8 KB (+0.05% 🔺)
@sentry/browser - Webpack (gzipped + minified)	19.71 KB (0%)
@sentry/browser - Webpack (minified)	64.15 KB (0%)
@sentry/react - Webpack (gzipped + minified)	19.73 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	44.02 KB (+0.12% 🔺)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	25.82 KB (+0.38% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.06 KB (+0.27% 🔺)