Add some additional flags to `svmkit` and `pulumi-svmkit`'s Agave validator

[alexanderguy]

Outcome

We need the following Agave flags:

• [X] extraFlags - for being able to add arbitrary flags we don't support directly yet.
• [X] --gossip-host
• [X] --allow-private-addr

[linear[bot]]

KIT-49 Add some additional flags to `svmkit` and `pulumi-svmkit`'s Agave validator