[ENH] 2 new environmental variables required for nexus

[vaimdev]

Nexus will be added 2 new environmental variable

1. GRAPHISTRY_NEXUS_ENCRYPTION_KEYS - type: base64
2. GRAPHISTRY_NEXUS_SIGNING_KEY - uuid

related PR
https://github.com/graphistry/graphistry/pull/2061

They are used for encryption & data signing purposes, and should be unique per installation.

[aucahuasi]

@vaimdev How and when we will generate those values?

[vaimdev]

when a new nexus is first created, only once per nexus, because once generated, it cannot be changed.

Useful in the scenario user spin a Graphistry in AWS market or Azure market.

[aucahuasi]

Got it, thank you!