Add neon_cli_version handler

[vasiliy-zaznobin]

The proxy should return the neon-cli version for the "neon_cli_version" (like "web3_clientVersion") request in format
Neon-cli/v<NEON_CLI_PKG_VERSION>-<NEON_CLI_REVISION>
For example:
Neon-cli/v0.4.0-rc0-abc26675a5f979eeeeff3c886ebf4fcd310ce064