[cronet_http] Add DNS configuration options to CronetEngine.build

[fryette]

Expose Cronet DnsOptions through new CronetEngine.build parameters: useBuiltInDnsResolver, enableStaleDns, persistHostCache and persistHostCachePeriod.

Cronet enables QUIC by default, and with QUIC enabled it resolves hosts through its built-in DNS resolver. On some cellular networks and in background isolates (e.g. WorkManager) that resolver fails with ERROR_HOSTNAME_NOT_RESOLVED while the system resolver works. useBuiltInDnsResolver: false forces the system resolver; enableStaleDns/persistHostCache allow a freshly created engine to serve recently used hosts from a persisted cache.

setDnsOptions is available on the stable CronetEngine.Builder in both cronet-api artifacts the plugin ships (play-services and embedded), and the Java API layer transparently falls back to an experimental-options JSON patch on implementations that do not support DnsOptions natively, so no experimental bindings are required.

Helps with #1217

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[fryette]

@brianquinlan will you be able to provide review?

[brianquinlan]

I'll look at this on Monday.

[fryette]

@brianquinlan any chance for merge?

[fryette]

@brianquinlan fixed everything, please check once you have time, thanks!

[brianquinlan]

Thank you so much!