[java-logging-logback] Use LogSplit for logs exceeding maximum size

**Is your feature request related to a problem? Please describe.**
In our projects we are using LoggingAppender to write logs in GKE. Some of the logs are bigger than the limit and they are rejected:
`ERROR: onFailure exception: com.google.cloud.logging.LoggingException: io.grpc.StatusRuntimeException: INVALID_ARGUMENT: Log entry with size 293.5K exceeds maximum size of 256.0K`

**Describe the solution you'd like**
LoggingAppender should split the logs exceeding limit and use "split" field described in google.logging.v2 API.

**Additional context**
LogEntry definition with `split` field:
https://cloud.google.com/logging/docs/reference/v2/rpc/google.logging.v2#logentry
LogSplit definition:
https://cloud.google.com/logging/docs/reference/v2/rpc/google.logging.v2#google.logging.v2.LogSplit
Field definition in proto file:
https://github.com/googleapis/googleapis/blob/master/google/logging/v2/log_entry.proto#L183