Document property indirection for passwords coming from Vault - Fixes #3279

[vsevel]

This provides documentation (and a test) on the ability to store a password in vault, and refer to it anywhere in the configuration using property indirection.

This was made possible thanks to #3937.

This solves for instance #3279, assuming Vault is an option in the user's environment.

cc @sberyozkin

[sberyozkin]

@vsevel IMHO it is top class, the extensions can continue using their own properties to work with Vault :-)

[vsevel]

@sberyozkin thanks for merging; you were too fast for me ;) I just finished running a full example on mail+vault quickstart using the property indirection.

here is the config I used:

quarkus.vault.secret-config-kv-path=myapps/vault-quickstart/config
        
quarkus.mailer.from=test@quarkus.io
quarkus.mailer.host=localhost
quarkus.mailer.port=1025
quarkus.mailer.ssl=false
quarkus.mailer.username=foo
quarkus.mailer.password=${a-private-key}

and for the smtp server:

docker run -p 1080:1080 -p 1025:1025 djfarrelly/maildev --incoming-user foo --incoming-pass 123456 -v

the extensions can continue using their own properties to work with Vault :-)

indeed. it is pretty nice.
(except for the lack of property value refresh)