feat(ext.bridge): add bridge.Context as a Union class

[OmLanke]

Summary

Adds a shortcut Union class for BridgeExtContext, and BridgeApplicationContext.
Since bridge.BridgeContext is an ABC, using that as a typehint doesn't display the attributes and methods available.

Current Workaround

Using the long form everywhere

@bot.bridge_command()
async def ping(ctx: bridge.BridgeExtContext | bridge.BridgeApplicationContext):
  ...

Desired Solution

Using the Union class

@bot.bridge_command()
async def ping(ctx: bridge.Context):
  ...

Information

• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed).
• This PR is not a code change (e.g. documentation, README, typehinting,
  examples, ...).

Checklist

• I have searched the open pull requests for duplicates.
• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why.
• I have updated the changelog to include these changes.