Implement profiles to specify scicat instances more easily

[jl-wynen]

Fixes #37

How to use profiles

With this PR, we can connect to ESS SciCat using

client = Client.from_token("ess", token=<TOKEN>)

However, this will only work on a non-ess machine if the user has their SSH agent set up properly. If they don't, they still need to provide a file transfer manually. E.g.:

transfer = SFTPFileTransfer(
    host="login.esss.dk",
    username=<USERNAME>,
    password=<PASSWORD>,
)
client = Client.from_token("ess", token=<TOKEN>, file_transfer=transfer)

I am happy to add profiles for other instances to the package if requested!

Loading profiles from files

This PR includes the option of loading profiles from TOML files. But this is very limited at the moment and cannot load file transfers. This is because I am unsure how useful this actually is. So reviewer, please let me know if you think that this is valuable enough to expand support. Or if it should be removed because it is too complicated.

[jl-wynen]

There is an open question: How an a user authenticate with SFTP if they don't use an SSH agent? The file transfer allows passing username and password. But the current profile implementation does not support that. I think this is required for many people. But it is tricky because the scicat credentials can be different than the sftp credentials. Otherwise, we could forward arguments from Client.from_credentials.

[jl-wynen]

Let's postpone passing credentials to the file transfer to later. This is a larger issue that should not block merging this PR.