[Feature Request] Allow configuring repository options for copy target.

[itzTheMeow]

i have a "main" backup on backblaze b2 and a redundancy one on s3 glacier deep archive.
however if i want to rustic copy a snapshot from one repo to the other. they both need the environment variables, since it has to connect to both b2 and s3 and they have different env vars
im trying to avoid storing secrets in text files, so i have env files with the OPENDAL_* environment variables in it for backblaze, and im using the 1password cli's run command to inject the secrets into the env vars
but s3 glacier also needs environment variables to connect.

one possibility was TARGET_OPENDAL_* environment variables for configuring the copy target
discussed on discord: https://discord.com/channels/1101748142694748181/1101748180007264276/1464154081089032318
im not sure how this would work for multiple targets though

another idea i had was the ability to get the opendal config options from a command
this would work for multiple targets