Add optional Req HTTPClient (and make it the default)

[ryanwinchester]

Summary

• Add a Req-based HTTP client
• Make both Req and Hackney optional dependencies.
• Automatically select an available HTTP client, preferring Req when both are installed.

Extras

• CI and version updates
• Fix a bad pattern-match

HTTP client selection

By default, Tzdata now selects its HTTP client in this order:

1. Req
2. Hackney
3. An actionable runtime error if neither dependency is available

Users can still explicitly configure an adapter or provide a custom implementation of the Tzdata.HTTPClient behaviour:

config :tzdata, :http_client, Tzdata.HTTPClient.Hackney

No HTTP client dependency is required when automatic timezone data updates are disabled.

Closes #174
Closes #165
Closes #162

[noteven]

Highly support this, tzdata is pushing an extremely out-of-date transitive dependency on hackney onto projects that leaves them open to CVEs of high severity.