chore: Add Splunk HEC indexer acknowledgement RFC (#9863)

* Add Splunk HEC indexer acknowledgement RFC

Signed-off-by: 001wwang <will.wang@datadoghq.com>

* Remove trailing spaces

Signed-off-by: 001wwang <will.wang@datadoghq.com>

* Annotate code blocks

Signed-off-by: 001wwang <will.wang@datadoghq.com>

* Minor rewording and formatting

Signed-off-by: 001wwang <will.wang@datadoghq.com>

* Update splunk_hec sinks implementation with oneshot channel design

Signed-off-by: 001wwang <will.wang@datadoghq.com>

* Add user configurable settings for sink acknowledgements

Signed-off-by: 001wwang <will.wang@datadoghq.com>

* Add channel related settings to splunk_hec source user experience

Signed-off-by: 001wwang <will.wang@datadoghq.com>

* Add implementation details around channel behavior

Signed-off-by: 001wwang <will.wang@datadoghq.com>

* Reword to be more specific and fix errors

Signed-off-by: 001wwang <will.wang@datadoghq.com>

* Specify dynamic allocation for bitvecs

Signed-off-by: 001wwang <will.wang@datadoghq.com>

* Add details for implementing max_pending_acks behavior

Signed-off-by: 001wwang <will.wang@datadoghq.com>

* Clean up formatting

Signed-off-by: 001wwang <will.wang@datadoghq.com>

* Add future improvements section

Signed-off-by: 001wwang <will.wang@datadoghq.com>

* Capitalize Vector

Signed-off-by: 001wwang <will.wang@datadoghq.com>

* Use roaring bitmap instead of bitvec

Signed-off-by: 001wwang <will.wang@datadoghq.com>

* Update Mutex to RwLock for channel implementation

Signed-off-by: 001wwang <will.wang@datadoghq.com>