[rsproxy] Re-pack ResultStore upload requests

Adapt the UploadRequests from the input stream to the ResultStore API:
a CreateInvocationRequest, followed by any number of
UploadBatchRequests.

cmd/rsproxy/main.go:
remove unused flags, fields from partialInvocation() are be merged with
the Invocation data coming from the input file/fifo to form
a complete CreateInvocationRequest.

internal/pkg/rsproxy/resultstore.go:
The main Run() function is now responsible for splitting the
initial CreateInvocation request from the UploadRequest stream.
Forward remaining requests to the internal request queue,
where they will be assembled into batches for the UploadBatch RPC.

internal/pkg/rsproxy/server.go:
Move ResultStore-related logic into ResultStore.
Server now just forwards an io.Reader directly to the
ResultStoreUploader via LoadUploadRequestsFromFile.
Run() is now much simplified.

Test: Running rsproxy alongside ninja connected by temporary mkfifo.
Bug: 390427892
Change-Id: Ia596c5917f21fca3c51ccc388ffdc3d15e0cadc2
Reviewed-on: https://fuchsia-review.googlesource.com/c/rsclient/+/1342064
Commit-Queue: David Fang <fangism@google.com>
Reviewed-by: Jay Zhuang <jayzhuang@google.com>