all: confirm and document specifications of all RPC APIs

[ijsong]

Varlog's daemons, such as varlogadm, varlogmr, and varlogsn, rely heavily on gRPC. However, we have not yet established any specifications or requirements for them. Currently, we are implementing them in a loose manner, for example, we don't have any explicit specification regarding the idempotence of MR's APIs. Therefore, it is necessary to establish clear requirements and specifications for the RPCs and document them. This will facilitate support for multi-language clients.

• admin: add specifications of varlogadm RPC #657
• metarepos: add specifications of varlogmr RPC #658
• storagenode: add specifications of varlogsn RPC #659
• client: add specifications of client API #660