feat: add linux distributions to os context

[supervacuus]

Fixes #943.

This adds Linux distribution meta-data to the os context as follows:

Only name and version should be indexed.

Further steps required before we merge/release this:

• [x] update event payload docs for OS context: https://github.com/getsentry/develop/pull/1227
• [x] add definitions in relay: https://github.com/getsentry/relay/pull/3443
• [x] add indexing support for name and version: https://github.com/getsentry/sentry/pull/69865
• [ ] restructure event-payload to be flat (i.e. distribution_name instead of distribution.name, etc.)
• [ ] apply flat structure to develop-docs (+ fix the link in develop-docs back to a commit in master: https://github.com/getsentry/sentry-docs/pull/10768)
• [ ] apply flat structure to relay
• [ ] apply flat structure to indexing support