[DE-69] jwt authentication

[rashtao]

Implement support to JWT authentication for VST and HTTP connections, for sync and async variants.

Create new API to:

set the JWT token in configuration file (sync and async)

set the JWT in the driver builder (sync and async)

update the JWT at runtime (sync and async)

NOTE: in VST the JWT is checked only in the initial authentication request. Any follow-up requests over the same connection will not re-validate the JWT, therefore there is no need for the driver to close VST connections whose authentication JWT has expired.

doc PR: arangodb/docs#856

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fixes #335