[raft] Protobuf encoding of proposals

[MariemBaccari]

This PR extracts the work of #1587 to implement protobuf based encoding of the proposal envelope (part of #1709). The previous PR used binary encoding but protobuf is a simplification that only adds a few bytes to each payload.
Follow-up PRs will implement compression and custom encoding for the payload values which will further reduce the payload size.

A small experiment shows the size difference between the json and protobuf encodings:

Case	Cells	Payload (B)	JSON envelope (B)	Protobuf envelope (B)	Saved
RID delete ISA	-	75	276	145	47.5%
SCD delete OIR	-	213	461	285	38.2%
RID create ISA	1	275	544	346	36.4%
RID create ISA	10	455	784	526	32.9%
RID create ISA	100	2255	3184	2326	26.9%
RID create ISA	1000	20255	27184	20327	25.2%
RID create subscription	1	310	579	368	36.4%
RID create subscription	10	490	819	548	33.1%
RID create subscription	100	2290	3219	2348	27.1%
RID create subscription	1000	20290	27219	20349	25.2%
SCD create subscription	1	384	675	442	34.5%
SCD create subscription	10	564	915	622	32.0%
SCD create subscription	100	2364	3315	2422	26.9%
SCD create subscription	1000	20364	27315	20423	25.2%
SCD create constraint	1	296	566	361	36.2%
SCD create constraint	10	476	806	541	32.9%
SCD create constraint	100	2276	3206	2341	27.0%
SCD create constraint	1000	20276	27206	20342	25.2%
SCD create OIR	1	466	801	538	32.8%
SCD create OIR	10	646	1041	718	31.0%
SCD create OIR	100	2446	3441	2518	26.8%
SCD create OIR	1000	20446	27441	20519	25.2%