Searchable custom context

### Problem Statement

It's not 100% clear whether this is currently possible or not, but if so, I could not discover how —

It would be great if Sentry's issue search would allow us to search issued based on structured custom context values. Without this ability, the developer is incentivized to neglect structured custom contexts for tags instead, which are searchable.

In addition, custom data associated with the sentry user in the SDK similarly ought to be surfaced.

### Solution Brainstorm

eg.
```
....:
```
or
```
context.....:
```

For data associated to the sentry user, eg. through `sentryUser.data = ["custom_key": "value"]`:
```
user.:
```
or
```
user.data.:
```

Since user-associated data appears in the issue event identically as a custom context named "User", perhaps the best consistency would be achieved with the combination:
```
....:
user.:
```

### Product Area

Issues