agent: Allow asynchronous unregistration.

[abezukor]

Pairing agents currently unregister themselves after the AgentHandle is dropped, but there is no way to know when the agent gets unregistered. That information is useful if you are trying to register a new agent, as registering a new agent will result in an error if the old pairing agent is still registered. This PR add a new async unregister method on AgentHandle that only returns after registration is completed.

I have tested this by registering a new agent immediately after awaiting the unregister function.

[abezukor]

CI failure is unrelated to this PR:

  Downloading crates ...
error: failed to download `getrandom v0.4.2`

Caused by:
  unable to get packages from source

Caused by:
  failed to download replaced source registry `crates-io`

Caused by:
  failed to parse manifest at `/home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/getrandom-0.4.2/Cargo.toml`

Caused by:
  feature `edition2024` is required

  The package requires the Cargo feature called `edition2024`, but that feature is not stabilized in this version of Cargo (1.75.0 (1d8b05cdd 2023-11-20)).

[abezukor]

CI failing due to #193