Introduce cache cap, backoff when retrying previously failed events

[kkostov]

This PR addresses a gap identified in #284 by capping the maximum number of events we keep in cache and introducing a back-off retry mechanism for previously failed events.

Users can configure both by overriding initialiser parameters on DefaultEventCache and DefaultEventTransmitter:

Parameter	Type	Default	Description
DefaultEventCache(cacheLimit:)	Int	10_000	Maximum events held in memory (FIFO)
DefaultEventTransmitter(transmitInterval:)	TimeInterval	10	Seconds between transmission attempts
DefaultEventTransmitter(maxBackoffInterval:)	TimeInterval	300	Upper bound for exponential backoff after consecutive failed batches

let cache = DefaultEventCache(cacheLimit: 5_000)
let transmitter = DefaultEventTransmitter(
    configuration: config,
    cache: cache,
    logger: DefaultLogger(),
    transmitInterval: 30,
    maxBackoffInterval: 600
)

Users can fully replace this implementation by providing their own EventCaching and EventTransmitting implementation