Add is connected status to skill context

[DavidMinarsch]

Proposed changes

Thi PR exposes the connection status to the skill context.

Fixes

It addresses #358.

Types of changes

What types of changes does your code introduce to agents-aea?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING doc
• I am making a pull request against the develop branch (left side). Also you should start your branch off our develop.
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have checked that code coverage does not decrease.
• I have checked that the documentation about the aea cli tool works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Further comments

In the future - once we have the multiplexer - we can collect each connection's status in a ConnectionStatuses object. Then we can also add additional information, like reconnection counts etc.

#358 asks for the ledger api status to also be exposed. However, the entire ledger api is accessible via the context so no further work is required there.