Allow a piece of Context to be "Promoted" to a Tag in Sentry UI

### Problem Statement

AFAIK currently there is no way to instruct Sentry to treat a piece of processed context as a Tag. In order to achieve this, SDK users must manually create a tag from some context as part of the API call.

For example: I want to see user `locale` information in Issue trends and create useful metrics for if all issue events are from the same locale.
To achieve this I would either need to add it to `beforeSend` or `configureScope`

```
options.beforeSend = { event ->
val deviceContext = event.contexts["device"] as? Map<*, *>
val locale = deviceContext?.get("locale") as? String
locale?.let { event.setTag("locale", it) }
event // Always return the event
}
```

```
Sentry.configureScope { scope ->
val deviceContext = scope.getContexts()["device"] as? Map<*, *>
val locale = deviceContext?.get("locale") as? String
locale?.let { scope.setTag("locale", it) }
```

This seems particularly cumbersome and a maintenance burden when this context is already present in the data Sentry receives.
I would request that a piece of Context be 'promotable' to a Tag, either in the Contexts UI or Issues -> Configure -> Tags perhaps.

### Solution Brainstorm

_No response_

### Are you willing to submit a PR?

_No response_

The issue does not name implementation files, tests, or an entry point. Start by tracing the Sentry UI's Contexts view and Issues → Configure → Tags flow, then determine the product and design scope for promoting a context field. Done should include an agreed interaction, supported context values, and verification that the promoted value is available as a tag for trends and metrics.

Written by the indexing model from the issue text.