chore: make finch optional

[solnic]

Address #967 (comment)

#skip-changelog

[sentry]

Bug: Making :finch optional without changing the default HTTP client from Sentry.FinchClient will cause a startup crash for users who don't have :finch installed.
_{Severity: CRITICAL}

Suggested Fix

To avoid this breaking change, either change the default HTTP client to one that doesn't depend on :finch, or add logic to the configuration validation to select a different default client if :finch is not available. Alternatively, revert making :finch optional or treat this as a major version bump with clear documentation about the new requirement.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: mix.exs#L104

Potential issue: The default HTTP client is set to `Sentry.FinchClient`. The application
startup logic checks if this client module is loaded and has a `child_spec/0` function.
Since `Sentry.FinchClient` is part of the `:sentry` application, these checks pass even
if the `:finch` library isn't installed. The code then proceeds to call
`Sentry.FinchClient.child_spec/0`, which explicitly raises an error if `:finch` is not
available. This results in an unhandled exception that crashes the entire application on
startup for any user who updates to this version and does not have `:finch` in their
dependencies.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[solnic]

It's fine. It's documented that you have to install finch yourself (by adding it as a dep).